
Class __QKq o» 

Book ^£<*3_ 

GoipglrtN? 

COPYRIGHT DEPOSIT. 



PLANT PHYSIOLOGY 



AND 



ECOLOGY 



FREDERIC EDWARD CLEMENTS, Ph.D. 

Professor of Botany in the University of Minnesota 



WITH 125 ILLUSTRATIONS 




NEW YORK 
HENRY HOLT AND COMPANY 

1907 



QvWo 



U8RA8Y of CONGRESS 
Two Gogies Received 

AUG 2 ,90f 

Cenynght r ntry 

CLASS A XXc, No: 

COPY 8. 



Copyright, 1907, 

BY 

HENRY HOLT AND COMPANY 



ROBERT DRUMMOND COMPANY, PRINTERS. NEW YORK 



PREFACE 

The point of view and the methods of study first advanced in 
" Research Methods in Ecology" have proved so satisfactory in 
teaching as to make it desirable to embody them in a text-book. 
The present text has been based largely upon " Research Methods," 
though most of the matter is new or rewritten. The manner of 
treatment is essentially the same, but the subject-matter has been 
rearranged and broken up into a larger number of chapters. The 
plant is first considered as an individual, with respect to factor, 
function, and form, and then as a member of a plant group or 
formation. 

The reasons for regarding ecology and physiology as essentially 
the same have been given elsewhere, and need not be repeated 
here. An endeavor has been made to give the various parts of this 
vast field their proper importance. Since ecology and physiology 
are merged, it is manifestly impossible to give to either what would 
be regarded as a complete treatment by a specialist in either line. 
No attempt has been made to touch all the points in each, but it is 
thought that nothing really fundamental has been omitted. 

The book is intended for use with classes in second-year botany 
in college and university. In manuscript form, it has been in such 
use for two years with good results. Although the amount of 
laboratory and field work is large, it is possible to accomplish all of 
it in a course requiring 6-8 hours of laboratory time each week. 
This can be done only by careful planning on the part of both in- 
structor and student, and for this reason the following suggestions 
are offered as aids. 

The instructor will find it imperative to plan in advance for the 
experiments for the whole year, in order that plants may be ready 
as needed. Seeds and fruits for the study of migration should be 
collected in the fall. Shade tents, water-content series, and com- 



iv PREFACE 

petition cultures must also be prepared early in the year. Types 
of hydrophytes, xerophytes, etc., should be grown in the plant- 
house in so far as possible. Students should be instructed to make 
duplicate plantings of all plants to be used in order to make sure 
of an adequate supply at all times. It has also been found desira- 
ble to teach the students the use of the paraffin method of em- 
bedding tissues, thus saving much time and securing better results. 
The work on adaptation to water and light is best carried on in the 
form of joint experiments, in which each student is assigned a 
definite part. In the experiments much use has been made of the 
common sunflower. This is on account of the ease with which it 
may be secured and grown, but w T hen a larger choice is possible 
other plants may often be substituted to advantage. 

In just as far as possible, the work of the student should be 
among plants out-of-doors. This is imperative in the chapters on 
vegetation, and is very desirable in all cases where it is feasible, 
even in the study of plant functions. For vegetation work, the 
knowledge of the more important genera and species of the several 
formations is indispensable. If the student does not already have 
this knowledge, the names should be furnished him by any desira- 
ble method, without taking the time necessary for identification. 

A bibliography has not seemed necessary and has not been given. 
A fairly full list of the more important works is found in " Research 
Methods." Apart from the latter, Pfeffer's "Pflanzenphysiologie," 
MacDougaPs "Text-book of Plant Physiology," Sach's "Text-book 
of Botany/' Vines' "Lectures on the Physiology of Plants," and 
Kerner's " Pflanzenleben " have been frequently consulted in the 
preparation of the text. 

Grateful acknowledgment is made of the kindness of Dr. C. E. 
Bessey, Dr. D. T. MacDougal, and Dr. Edith Clements in reading 
and criticizing the text. The author is also indebted to Dr. Edith 
Clements for many drawings, and for the use of cuts from "The 
Relation of Leaf Structure to Physical Factors," and to Mr. R. J. 
Pool and Mr. A. W. Sampson for the loan of several photographs. 

Frederic Edward Clements. 
The University of Nebraska, 
March 1907. 



CONTENTS 

CHAPTER I 
STIMULUS AND RESPONSE 

PAGE 

1. Fundamental Relations 1 

2. The Nature of Stimuli 1 

3. The Kinds of Stimuli 3 

4. The Nature of Response 3 

5. Adjustment and Adaptation 4 

6. Kinds of Adjustment 5 

7. Normal and Abnormal Adjustment 5 

CHAPTER II 
THE WATER OF THE HABITAT 

8. Relation of the Plant to Water 7 

9. The Nature of Water Stimuli 7 

10. Water Content 8 

11. Influence of other Factors upon Water Content 9 

12. Available and Non-available Water Content 9 

13. Soil Samples 10 

14. Computation of Water Content 11 

15. Time of Water Content Readings 12 

16. Location of Readings 12 

17. Depth of Samples 12 

Experiment 1. Measurement of Water Content 13 

18. The Determination of Available Water 13 

19. Chresard of Habitats 14 

Experiment 2. Determination of Available Water 14 

Modifying Factors 

20. Influence of Soil upon Water Content 15 

21. Origin and Structure 16 

22. Water Capacity 17 

v 



vi CONTENTS 



PAGE 

23. Chemical Nature of Soils 18 

24. Air Content 19 

25. Determination of Soil Properties 19 

Experiment 3. Porosity and Rate of Evaporation 20 

26. Influence of Precipitation upon Water Content 21 

27. Measurement of Rainfall 21 

28. Physiography , 22 

29. The Influence of Slope 23 

30. The Influence of Surface 23 

31. The Influence of Climatic Factors 24 

32. Humidity 24 

33. Modifying Influence of Temperature and Wind 25 

34. Influence of Pressure and Physiographic Factors 26 

35. Effect of Climate and Habitat 27 

36. Measurement of Humidity 27 

37. Sling and Cog Psychrometer 28 

38. Making a Reading , 29 

39. Use of Humidity Tables 30 

Experiment 4- Measuring Humidity 30 

40. Method of Habitat Study 30 

41. Choice of Stations 31 

42. Constant Factors 31 

43. Simultaneous Readings 31 

44. Point and Hour Readings 33 

45. Records 33 

46. Kinds of Curves 34 

47. Combinations of Curves 34 

48. Plotting Curves 35 

49. Intervals for the Different Factors 36 

Experiment 5. Determining the Physical Factors of Habitats. . 36 



CHAPTER III 
ADJUSTMENT TO WATER 

50. Responses to Water Stimuli - 38 

Absorption 

51. General Relations 38 

52. The Form of Roots 39 

53. Primary Regions of the Root 39 

54. Detailed Structure 40 

55. Origin and Structure of Root-hairs 42 

56. Effect of Water Content upon Root-hairs and Roots 43 

Experiment 6. Structure of the Root and Formation of Root-hairs. 44 

Experiment 7. Hydrotropism 44 



CONTENTS vii 

1'AfiE 

57. Imbibition j I 

Experiment 8. Water of Imbibition 45 

58. Osmosis 45 

,">!). ( )smosis in Root-hairs 46 

60. Influence of Soluble Salts 47 

Experiment 9. Demonstration of Osmosis 47 

Experiment 10. The Effect of Soluble Salts 47 

61. Effect of Protoplasm upon the Absorption of Soluble Salts 48 

62. Diffusion 48 

Experiment 11. Diffusion in Liquids and in Tissues 49 

63. Turgidity 49 

Experiment 12. Demonstration of Turgidity 50 

Transport 

64. General Nature 50 

65. Types of Stem Structure 51 

66. Stem Structure of an Herbaceous Dicotyledon 52 

67. Stems of Monocotyledons 53 

68. Structure of Woody Stems 53 

69. Functions of the Stem 54 

Experiment 13. Structure of Stems 54 

70. The Upward Movement of the Water 54 

71. Causes of the Movement 55 

Experiment 14- Pathway and Rate of Movement 56 

Transpiration 

72. The Structure of a Representative Leaf 56 

73. The Chlorenchym 57 

74. The Reduced Bundles 58 

Experiment 15. Structure of a Leaf 58 

75. Diffusion in the Leaf 58 

76. Transpiring Surface 59 

Experiment 16. Measurement of the Actual Transpiring Surface . 59 

77. Structure and Position of Stomata 60 

78. The Functions of Stomata 61 

79. Movements of Guard-cells .• 61 

Experiment 17. Movement of Guard-cells 62 

Experiment 18. Position of Stomata and Water Loss 62 

80. The Influence of Physical Factors upon Transpiration 63 

81. The Measurement of Transpiration 64 

82. Measuring Transpiration in the Field ". 65 

Experiment 19. Influence of Factors upon the Rate of Transpira- 
tion 65 

83. The Amount of Transpiration in Plants 66 

84. Relation between Transpiration and Absorption 68 

85. Compensation for Increased Transpiration 68 



vi CONTENTS 



PAGE 

23. Chemical Nature of Soils 18 

24. Air Content 19 

25. Determination of Soil Properties 19 

Experiment 3. Porosity and Rate of Evaporation 20 

26. Influence of Precipitation upon Water Content 21 

27. Measurement of Rainfall 21 

28. Physiography 22 

29. The Influence of Slope 23 

30. The Influence of Surface 23 

31. The Influence of Climatic Factors 24 

32. Humidity 24 

33. Modifying Influence of Temperature and Wind 25 

34. Influence of Pressure and Physiographic Factors 26 

35. Effect of Climate and Habitat 27 

36. Measurement of Humidity 27 

37. Sling and Cog Psychrometer 28 

38. Making a Reading 29 

39. Use of Humidity Tables 30 

Experiment 4- Measuring Humidity 30 

40. Method of Habitat Study 30 

41. Choice of Stations 31 

42. Constant Factors 31 

43. Simultaneous Readings 31 

44. Point and Hour Readings 33 

45. Records 33 

46. Kinds of Curves 34 

47. Combinations of Curves 34 

48. Plotting Curves 35 

49. Intervals for the Different Factors 36 

Experiment 5. Determining the Physical Factors of Habitats. . 36 



CHAPTER III 
ADJUSTMENT TO WATER 

50. Responses to Water Stimuli - 38 

Absorption 

51. General Relations 38 

52. The Form of Roots 39 

53. Primary Regions of the Root 39 

54. Detailed Structure 40 

55. Origin and Structure of Root-hairs 42 

56. Effect of Water Content upon Root-hairs and Roots 43 

Experiment 6. Structure of the Root and Formation of Root-hairs. 44 

Experiment 7. Hydrotropism 44 



CONTENTS vii 

PAGE 

57. Imbibition 44 

Experiment 8. Water of Imbibition 15 

58. Osmosis 45 

59. Osmosis in Root-hairs 46 

GO. Influence of Soluble Salts 47 

Experiment 9. Demonstration of Osmosis 47 

Experiment 10. The Effect of Soluble Salts 47 

61. Effect of Protoplasm upon the Absorption of Soluble Salts 48 

62. Diffusion 48 

Experiment 11. Diffusion in Liquids and in Tissues 49 

63. Turgidity 49 

Experiment 12. Demonstration of Turgidity 50 

Transport 

64. General Nature 50 

65. Types of Stem Structure 51 

66. Stem Structure of an Herbaceous Dicotyledon 52 

67. Stems of Monocotyledons 53 

68. Structure of Woody Stems 53 

69. Functions of the Stem 54 

Experiment 13. Structure of Stems 54 

70. The Upward Movement of the Water 54 

71. Causes of the Movement 55 

Experiment 11^. Pathway and Rate of Movement 56 

Transpiration 

72. The Structure of a Representative Leaf 56 

73. The Chlorenchym 57 

74. The Reduced Bundles 58 

Experiment 15. Structure of a Leaf 58 

75. Diffusion in the Leaf 58 

76. Transpiring Surface 59 

Experiment 16. Measurement of the Actual Transpiring Surface. 59 

77. Structure and Position of Stomata 60 

78. The Functions of Stomata 61 

79. Movements of Guard-cells : 61 

Experiment 1 7. Movement of Guard-cells 62 

Experiment 18. Position of Stomata and Water Loss 62 

80. The Influence of Physical Factors upon Transpiration 63 

81. The Measurement of Transpiration 64 

82. Measuring Transpiration in the Field ". 65 

Experiment 19. Influence of Factors upon the Rate of Transpira- 
tion 65 

83. The Amount of Transpiration in Plants 66 

84. Relation between Transpiration and Absorption 68 

85. Compensation for Increased Transpiration 68 



vni CONTENTS 



86. Details of the Adjustment 69 

Experiment 20. Pathway of Adjustment 70 

CHAPTER IV 
ADJUSTMENT TO LIGHT 

87. Relations of the Plant to Light 71 

88. The Nature of Light Stimuli 72 

89. Measurement of Light 72 

90. Making a Standard 73 

91. Making Readings 74 

92. Comparison with the Standard 75 

93. Causes of Variation in Light Intensity 75 

94. The Effect of Time 75 

95. The Effect of Altitude 76 

Experiment 21. Measuring Light Intensity 77 

96. Reception and Absorption of Light 77 

97. The Amount Absorbed 77 

Experiment 22. Epidermis and Leaf Prints 78 

98. The Production of Chlorophyll 79 

99. The Nature of Chlorophyll 79 

100. The Influence of Darkness 80 

Experiment 23. Influence of Light and Darkness 81 

101. Photosynthesis 81 

102. Absorption and Diffusion of Carbon Dioxide 81 

103. Chemical Changes during Photosynthesis 82 

Experiment 24. Dependence of Photosynthesis upon Aeration 

and Light 83 

104. Measurement of Photosynthesis 83 

Experiment 25. Relation of Photosynthesis to Sun and Shade. . 83 

105. Translocation 83 

Experiment 26. Translocation 84 

106. Storage of Food Material 84 

Experiment 27. Storage Tissues 85 

107. Influence of Light upon the Number and Position of Chlorop lasts . 85 

Experiment 28. Arrangement of Chloroplasts 87 

108. Movement of Stems and Leaves 87 

109. Phototropism 87 

Experiment 29. Phototropic Movements 89 

110. Nyctotropism 89 

Experiment 30. Nyctotropic Movements 89 

CHAPTER V 
ADJUSTMENT TO TEMPERATURE 

111. Relations of Plants to Temperature 90 

112. The Measurement of Temperature 91 



CONTENTS ix 

PAGE 

1 13. Soil Temperatures 92 

114. Plant Temperatures 92 

Experiment 31. Temperatures of Plant and Habitat 93 

115. Variations of Temperature 94 

116. The Influence of Other Factors 95 

117. Favorable and Unfavorable Temperatures 95 

118. Freezing 96 

Experiment 32. Effects of Freezing 97 

119. The Sum of Temperatures 97 

120. Influence upon Vegetation 98 

121. Digestion 99 

122. Chemosynthesis of Digested Materials 100 

123. Respiration 101 

124. Fermentation 102 

125. Germination 102 

Experiment 33. Digestion and Respiration in Seeds 103 

126. Nutrition of Hysterophytes 104 

127. Kinds of Parasites 104 

Experiment SJf.. Nutrition of Representative Hysterophytes 105 

128. Growth 106 

129. Growth of Tissues and Organs 106 

Experiment 35. Regions of Growth 107 

130. Conditions that Influence Growth 108 

131. The Amount and Rate of Growth 108 

132. Regions of Greatest Growth in Various Organs 109 

Experiment 36. Influence of Temperature, Water and Light upon 

Regions of Growth 110 

133. Rhythm of Growth 110 

134. The Age of a Plant 112 

135. Reproduction 112 

136. Propagation 113 

137. Propagules of Flowering Plants 114 

138. Stems as Propagules 114 

Experiment 37. Propagules 115 

139. Sexual Reproduction 115 

140. Production of Pollen 116 

Experiment 38. Amount of Pollen 118 

141. Protection of Pollen 118 

142. Structural Protection 119 

143. Protection by Movement 120 

144. Seasonal Protection 121 

Experiment 39. Protection of Pollen 122 

145. Disposition of the Stamens and Pistils 122 

Experiment 4.0. Grouping of Stamens and Pistils 123 

146. Source and Destination of Pollen 124 

147. Cross-pollination 125 

148. Pollination by Insects 126 



x CONTENTS 

PAGE 

149. Self-pollination 127 

Experiment 41 . Pollination 128 

150. The Period of Flowering 128 

151. Time of Daily Flowering 130 

Experiment 42. Time of Flowering 131 

152. Fructification 131 

153. Fleshy Fruits 131 

154. Dry Fruits 132 

155. Movements of Fruits 132 

Experiment 43. Kinds of Fruits 133 

CHAPTER VI 
ADJUSTMENT TO GRAVITY, CONTACT AND SHOCK 

156. The Relation of the Plant to Gravity 135 

157 . Geotropism 135 

158. Cause and Reaction 137 

159. Region of Curvature 138 

160. Ecological Significance of Geotropism 139 

Experiment 44- Geotropism 139 

161. Response to Contact. 141 

Experiment 45- The Behavior of Tendrils 142 

162. Response to Shock 142 

Experiment 46. Response to Shock 143 

CHAPTER VII 
ADAPTATION TO WATER 

163. The Relation of Structures to Water 144 

164. Adaptation to a Small Water Supply 144 

165. Decrease of Water Loss through Leaf Position 145 

166. Decrease through the Rolling of the Leaf . 145 

167. Reduction of Leaf or Stem '. 147 

168. Changes of the Epidermal Cells 147 

169. Modifications of the Stomata 148 

170. Changes in the Chlorenchym 150 

171. Increase and Storage of Water Supply 151 

172. Adaptation to Excessive Water Supply 152 

Experiment 47 '■ Experimental Adaptation to Water 153 

173. Types of Plant Body 154 

174. Types Produced by Adaptation to Water 155 

175. General Features of Xerophytes 156 

176. Types of Leaf Xerophytes 156 

177. Normal Leaf Xerophytes 157 

178. Storage Leaves , 158 



CONTENTS xi 

PAGE 

179. Lanate Leaves 159 

Experiment 4.S. Study of Normal Leaf Xerophytes 159 

180. Other Leaf Xerophytes 159 

Experiment Jfi. Study of Xerophytic Leaves 162 

181. Stem Xerophytes 162 

182. Types of Stem Xerophytes 162 

Experiment 50. Form and Structure of Stem Xerophytes 164 

183. Mesophytes 164 

Experiment 51. Comparison of Mesophyte and Xerophyte. ... 165 

184. Hydrophytes 165 

185. Amphibious Plants 166 

Experiment 52. Structure of Amphibious Plants 167 

186. Floating Plants 167 

Experiment 53. Structure of Floating Plants 168 

187. Submerged Plants 168 

Experiment 54- Structure of Submerged Plants 168 

188. Bog Plants 168 

Experiment 55. Study of Water Content Types 170 

CHAPTER VIII 
ADAPTATION TO LIGHT 

189. The Relation of Organs to Light 171 

190. Influence of the Chloroplasts 171 

191. Modifications of the Chlorenchym 172 

192. Sponge Tissue 174 

193. Palisade Tissue 176 

194. Changes of the Epidermis 177 

195. The Form of Leaves 178 

196. Changes of Outline, Size and Thickness 178 

197. The Form of Stems 179 

Experiment 56. The Production of Adaptations to Light 181 

198. Types of Leaves as Determined by Light 182 

199. Sun Plants and Shade Plants 183 

Experiment 57. Sun and Shade Forms in Nature 184 

CHAPTER IX 
THE ORIGIN OF NEW FORMS 

200. The Law of Evolution 185 

201. Stability and Plasticity 185 

202. Constant and Inconstant Forms 186 

203. Origin by Descent before Darwin 187 

204. Darwin and the Origin of Species 188 

205. Evolution after Darwin 189 



xii CONTENTS 

PAGE 

206. Fundamental Methods of Evolution 191 

207. Origin by Adaptation 192 

208. Origin by Variation 195 

209. Origin by Mutation 196 

210. Origin by Hybridation 197 

Experiment 58. The Occurrence of New Forms in Nature 198 

211. Natural Selection 198 

212. Isolation 199 

213. Polygenesis 199 

214. Experimental Evolution 201 

CHAPTER X 
METHODS OF STUDYING VEGETATION 

215. The Study of Vegetation 202 

216. The Quadrat 202 

217. Kinds of Quadrats 203 

218. Marking out Quadrats 204 

219. The List Quadrat 204 

220. Abundance 205 

221. The Chart Quadrat 205 

222. Making Quadrat Charts 206 

223. The Permanent Quadrat 208 

224. The Denuded Quadrat 209 

225. Transects 210 

226. The Line Transect 210 

227. The Belt Transect 211 

228. The Migration Circle 212 

229. Formation Maps 213 

CHAPTER XI 
THE PLANT FORMATION 

230. The Nature of Formations 215 

231 . Recognition of Formations 216 

232. Relation between Habitat and Formation 218 

233. The Historical Factor 219 

234. Development and Structure 219 

235. Structure of the Formation 221 

236. Facies 221 

237. Principal and Secondary Species 222 

238. Aspects 223 

Experiment 59. Study of Abundance and of Aspects 224 

239. The Parts of a Formation: the Consocies 225 

240. The Society 226 



CONTEXTS Xlli 

PAGE 

241. The Community 227 

242. The Family 228 

Experiment 60. The Structure of a Formation 229 

243. Layers 229 

Experiment 61. Layered Formations 230 

244. Classification 230 

245. Classification by Habitats 231 

246. Types of Formations 232 

247. Developmental or Physiographic Classification 233 

248. Regional Classification 234 

249. Open and Closed Formations 235 

250. Mixed Formations 235 

Experiment 62. Comparison of Formations 236 



CHAPTER XII 
AGGREGATION AND MIGRATION 

251. Aggregation 237 

252. Simple Aggregation 237 

253. Mixed Aggregation 239 

Experiment 63. Study of Families and Communities 240 

254. Migration 240 

255. Mobility 241 

256. Organs of Dissemination 241 

257. Modifications for Migration 242 

258. Influence of Seed Production 243 

259. Position of Disseminules 244 

260. The Agents of Migration 244 

261. The Work of Migration Agents 248 

Experiment 64- Modifications for Migration 249 

262. The Direction of Migration 249 

Experiment 65. Amount and Direction of Migration 250 

CHAPTER XIII 
COMPETITION AND ECESIS 

263. Competition 251 

264. The Struggle for Existence 251 

265. The Nature of Competition 252 

266. The Factors Involved 253 

267. Competition for Water and Light 253 

268. Competition between Parents and Offspring 254 

269. Competition between Different Species 255 

270. Influence of Vegetation Form and Habitat Form 256 

271. The Effect of Position 256 



xiv CONTENTS 

PAGE 

272. Vegetation Pressure 257 

273. The Results of Competition 258 

274. The Study of Competition 259 

275. Competition Cultures 259 

276. Competition Quadrats 261 

Experiment 66. Competition 261 

277. Ecesis 261 

278. The Factors in Ecesis 262 

279. Germination of the Seed 263 

280. The Effect of Habitat 264 

281. Adjustment to the Habitat 265 

Experiment 67. Influence of Habitat Form upon Ecesis 265 

282. Barriers 265 

283. Physical and Biological Barriers 266 

284. Influence of Barriers 266 

285. Distance 267 

286. Endemism 268 

Experiment 68. Barriers and Endemism 269 

CHAPTER XIV 

INVASION AND SUCCESSION 

287. Invasion 270 

288. The Manner of Invasion 270 

289. Invasion at Different Levels 272 

290. Kinds of Invasion 272 

291. Indigenous and Derived Species 273 

Experiment 69. Invasion 273 

292. Succession 273 

293. Kinds of Succession 274 

294. Primary Successions 275 

295. Succession in Colluvial Soils 276 

296. Succession in Alluvial Soils 277 

297. Succession in iEolian Soils 278 

298. Secondary Successions 278 

299. Succession in Eroded Soils 279 

300. Succession in Flooded Soils 280 

301. Successions Due to Man 280 

302. Successions in Burned Areas 281 

303. Succession in Lumbered Areas 282 

304. Succession by Cultivation 282 

305. Reactions of Plants upon the Habitat 282 

306. The Laws of Succession 284 

307. The Study of Successions 286 

308. Method of Alternating Areas 286 

309. The Relict Method 287 

Experiment 70. The Study of a Secondary Succession 288 



CONTENTS xv 

CHAPTER XV 
ALTERNATION AND ZONATION 

PAGE 

310. The Relation between Alternation and Zonation 289 

311. Alternation 289 

312. Causes of Alternation 289 

313. Alternation Due to Ecesis 290 

314. Alternation Due to Competition 291 

315. Kinds of Alternation 291 

316. Normal Alternation of Fades, Consocies, etc 292 

317. Normal Alternation of Species 293 

318. Numerical Alternation 293 

319. Corresponsive Alternation 294 

Experiment 71. Alternation of Species 294 

320. Zonation 294 

321. Zones Due to Growth 295 

322. Zones Due to Migration and Ecesis 296 

323. Zones Due to Reaction 296 

324. Zones Due to Physical Factors 297 

325. Physiographic Symmetry . v 298 

326. Symmetry in Vegetation 298 

327. Kinds of Zonation 299 

328. Radial Zonation 299 

329. Bilateral Zonation 300 

330. Vertical Zonation 301 

331. Vegetation Zones 302 

Experiment 72. Zonation of Pond and Meadow Formations 303 

Index 305 



PLANT PHYSIOLOGY AND ECOLOGY 

CHAPTER I 
STIMULUS AND RESPONSE 

i. Fundamental relations. A plant is an organism capable 
of nourishing itself under the control of external conditions, and 
of modifying its form and structure in accordance with this fact. 
Hence the most important matter in the study of plants as living 
things is to find out how the making and the using of food take 
place, and how the carrying out of these processes affects the 
structure of the plant. In seeking to explain the behavior of 
the living plant, i.e., its activities or functions, the first need is 
to discover the external forces that control it. We must next 
determine the effects which these produce. These are first seen 
in the functions of the plant, and in some cases they become evi- 
dent here alone. As a rule, however, many of them appear sooner 
or later as a change in the minute structure or form of the plant. 
The proper task of physiology is the study of the external factors 
of the environment or habitat in which the plant lives, and of 
the activities and structures which these factors call forth. The 
former are causes, the latter effects. The sequence of study is 
consequently factor, function, and form, and the primary object 
to discover the nature and amount of this fundamental connec- 
tion between the causative factors and the resulting functions 
and forms. 

Physiology was originally understood to' be an inquiry into 
the origin and nature of plants. This is the view that pervades 
the following pages, and in accordance with this the subject-mat- 
ter of ecology is merged with that of physiology. 

2. The nature of stimuli. Any factor of the habitat that 
produces a change in the functions of a plant is a stimulus. The 



2 PLANT PHYSIOLOGY AND ECOLOGY 

real test of the latter is therefore furnished by the plant, since 
the presence of a stimulus can only be ascertained by the re- 
sponse made by the plant. Moreover, while it is possible for 
the effect of a stimulus to remain invisible or latent for a time, 
a factor which works in this way can never be recognized as a 
stimulus until its effect becomes apparent. Stimulus and re- 
sponse are consequently not only inseparably connected, but the 
latter is the only obtainable evidence of the action of habitat 
factors. Since plants grow constantly under slight fluctuations 
in the habitat, it has come about that they do not respond to 
minute differences of factors. Living plants are in constant 
response to stimuli, and they are stimulated anew only by an in- 
crease or decrease in the factor sufficient to bring about an appre- 
ciable change in a function. Sometimes the total withdrawal 
of a factor acts as a profound stimulus, as in the case of a plant 
placed in darkness. The nature of the plant itself is of the utmost 
importance in determining what differences are sufficient to con- 
stitute stimuli. A species whose characters have been fixed by 
heredity responds much less readily to external factors than does 
one in which the structures are variable or plastic. In other 
words, a difference sufficient to produce a change in the latter 
has no effect upon the former. Such a difference constitutes a 
stimulus for the one, but not for the other. Thus, while light 
acts as a stimulus to all green plants, a certain change in the 
intensity of the light is a stimulus only to those plants that are 
plastic enough to show a response to it. 

It has been the practice to distinguish between the tonic action 
of external factors, such, for example, as that of light upon the 
chloroplast, and the stimulatory action of such forces, as seen 
in the bending of leaves toward the light, or the movement of 
sensitive leaves in response to a shock. In the one case the 
energy of the impinging factors results in an immediate and usu- 
ally proportionate amount of work being done. In the other 
this factor brings about the release of stored-up energy in the 
plant, which in many instances results in a disproportionate 
amount of work. However, a careful analysis of these two proc- 
esses shows that at the bottom they are essentially the same. 
Furthermore, they are seen to differ only in degree, and not in 
kind, when one examines the many processes intermediate between 
the two. 



STIMULUS AND RESPONSE 6 

3. The kinds of stimuli. The simplest grouping of stimuli 

is with respect to the force concerned. The factors of a habitat 
are water, soluble salts, humidity, light, temperature, wind, soil, 
pressure, physiography, gravity, polarity, and biotic factors. Cer- 
tain ones of these, namely, soil, physiography, pressure, and 
biotic factors, can act upon plants only through the action of 
other factors, as a rule. For example, the wind normally influ- 
ences the plant only through humidity, and the soil through 
water content. Since a stimulus can be determined only by 
the response of the plant to it, only those factors that act imme- 
diately upon a function can be termed stimuli. These are the 
universal forces, gravity and polarity, and the physical factors, 
water, soluble salts, humidity, light, and temperature. With 
respect to certain mechanical effects, wind may also act as a 
stimulus, and the same is often true of biotic factors in the case 
of sensitive, insectivorous, and gall-producing plants. Stimuli are 
often distinguished as internal and external, but the distinction 
is of little value. This is due to the fact that internal stimuli 
are obscure in nature and effect: it is not improbable that they 
are merely the latent results of external stimuli. In any event, 
little can be done with them until more is known of the precise 
action of external stimuli. It is with the latter alone that our 
present study is concerned. 

4. The nature of response. Plants seem to have no special sense- 
organs for perceiving stimuli, and no definite sensory tracts for 
transmitting them. Consequently an external stimulus acting 
upon a plant is ordinarily converted into a response at once. 
The latter, as a rule, becomes evident immediately. In many 
cases some time elapses before the final response becomes visible, 
and in rare instances the response remains latent or impercepti- 
ble. A marked decrease in humidity calls forth an immediate 
increase in the amount of water evaporated from the leaf, but 
a final response is seen in the closing of the stomata. The re- 
sponse to decreased light, on the contrary, is much less rapid and 
obvious. This difference in behavior is largely due to the func- 
tional response being more marked and more easily perceptible 
in the first case. 

The first response of a plant to a stimulus is always functional. 
The nature and intensity of the stimulus determine whether this 
is followed by a structural response also. The amount of re- 



4 PLANT PHYSIOLOGY AND ECOLOGY 

sponse is dependent upon the intensity of the stimulus , and it 
is in many cases proportional to it. The same stimulus may 
not produce the same response in two different species, or neces- 
sarily in two plants of the same species. It does have this effect 
in individuals and species that are equally plastic. The study 
of response is facilitated by distinguishing two kinds, viz., func- 
tional and structural. Many reactions to stimuli are functional 
alone. In a large number of cases a structural change also occurs, 
and this is the rule when the functional change is pronounced. 
Consequently, it becomes convenient to distinguish functional 
response as adjustment, and structural response as adaptation. 

5. Adjustment and adaptation. The adjustment of a plant 
to the stimuli of its habitat is taking place constantly. It is 
seen daily in the processes of nutrition and growth. As long as 
the stimuli are normal for the habitat, the adjustment of the 
plant is restricted to its ordinary activities. But when the stimuli 
become unusual in amount or in kind, either by a change of habitat 
or by a modification within it, the consequent adjustment becomes 
more evident, and is then usually recorded in the plant's struc- 
ture. Adjustment may be expressed in the movement of parts 
or organs, such as the closing of stomata or a change in the posi- 
tion of leaves, or in growth or modification of structure. Slight 
or periodic adjustment usually concerns function alone. Adjust- 
ment is profoundly affected by the nature of the factor, and is 
in direct relation to the intensity of the latter. Adaptation com- 
prises all structural changes resulting from adjustment. It in- 
cludes both growth and modification. The latter is really growth 
in response to unusual stimuli, a fact that furnishes the clue to 
all evolution. Growth is periodic and quantitative: it is the 
result of the normal and continuous adjustment of the plant to 
the stimuli of its own habitat. On the contrary, modification 
is relatively permanent and qualitative: it is the response to 
stimuli of an unusual kind or intensity. A good knowledge of 
the way in which growth occurs is indispensable to the under- 
standing of modification. In endeavoring to find the connection 
between habitat and plant, however, it is in the modification 
of the plant and not so much in its growth that the significant 
responses to stimuli are to be sought. 

In the following survey of the relation between the stimuli, 
functions, and structures of the plant, the physical factors of the 



STIMULUS AND RESPONSE 5 

habitat and the functional responses to them are considered 
under adjustment. Growth is also placed here both for con- 
venience and for the reason that it leads logically to the study 
of modifications. In consequence, the treatment of adaotation is 
practically confined to modifications of structure. 

6. Kinds of adjustment. With respect to the factor con- 
cerned, the functional responses of the plant are distinguished 
primarily as adjustment to water, light, or temperature. Re- 
sponses to soluble salts are properly considered under water, while 
the direct changes due to wind usually affect the form of the 
plant alone. The response to gravity is so universal, final, and 
absolute that it hardly falls within adjustment proper. Indi- 
rect factors, i.e., such as soil, wind, rainfall, pressure, and physi- 
ography, which can affect a function only by acting upon another 
or direct factor, do not properly produce response, but the change 
resulting from their influence is to be ascribed to the direct factor 
concerned. For example, the effects of soil, physiography, wind, 
and pressure are chiefly to be sought under adjustment to water, 
because of their action upon water content or upon humidity. 

7. Normal and abnormal adjustment. The unusual stimuli 
resulting from a greatly changed habitat or from a new one pro- 
duce an unusual or abnormal response in function and often 
in form. Adjustment is consequently to be regarded as normal 
or abnormal. Normal adjustment is characteristic of a plant 
that passes from youth to maturity in its own habitat. The 
functions are carried on in the manner usual to the species, and 
there is in consequence no modification of structure. Abnormal 
adjustment occurs in those plants that migrate into a new or 
different habitat, or those whose habitat is seriously changed. 
It is characterized, as a rule, by profound disturbance of function, 
though the latter clearly depends upon the intensity of the change. 
The most familiar cases of abnormal response are due to biotic 
factors, particularly parasitic fungi and insects. In most in- 
stances of this sort, the disturbance is merely functional, but 
often also the change in function is followed by a modification 
in growth or structure, as in the " cedar apples" and "witches, 
brooms" produced by rusts, and in the galls due to insects. 

A plant acted upon by a parasitic fungus or insect is said to 
be in a pathological condition. The study of the effect of the 
parasite upon the host-plant is called pathology, and it is regarded 



6 PLANT PHYSIOLOGY AND ECOLOGY 

as a subdivision of physiology. A plant that is more or less hin- 
dered in carrying out its usual functions by the presence of a fungus 
exhibits abnormal adjustment due to a biotic factor of the habitat. 
A sun plant that finds itself placed in the shade has likewise to 
adjust itself to light stimuli that are abnormal to it. During the 
period of adjustment it also is in a pathological condition. In 
both cases the adjustment must be successfully carried out or the 
plant dies. Consequently normal functioning is physiological and 
abnormal functioning is pathological. There is clearly no hard- 
and-fast line between the two, since any plant is acted upon by 
abnormal stimuli while it is getting established in a new habitat, 
but these same stimuli become entirely normal when the plant has 
become adapted to them. In studying the behavior of plants, it 
is both illogical and inconvenient to separate the normal and the 
abnormal. In the practical study of specific plant diseases, such 
separation is a matter of convenience, but in an elementary treat- 
ment it is undesirable to distinguish pathology from physiology. 



CHAPTER II 
THE WATER OF THE HABITAT 

8. Relation of the plant to water. The responses of the plant 
to the water of its habitat are so numerous and so essential that 
water must be regarded as the most important of all factors which 
affect the plant. This is emphasized by the fact that practically 
all indirect factors, i.e., soil, wind, etc., can influence the plant only 
through their action upon water. Water is no more indispensable 
to ordinary plants than is light or temperature, since a green plant 
can not live and function if any of these is lacking. It is proper to 
speak of it as more important, for the reason that water is the 
immediate cause of a larger number of vital functions. Perhaps 
the greatest value of water to the plant lies in its use as food. In 
addition it is the vehicle by which solid foods, i.e., soluble salts, are 
taken from the soil, and gases, carbon dioxide and oxygen, from the 
air, and by which the foods made by the leaves are carried to all 
parts of the plant. It is water that causes the stretching of the 
cell wall by which growth is made possible, and it also gives the 
rigidity so essential to stems of herbaceous plants. As a factor of 
the habitat, though not as a stimulus, water is an important agency 
in the reproduction of mosses and ferns and in the distribution of 
the plant body, or seeds of water plants. In the form of humidity, 
water regulates the loss of water from leaves. Finally, as is to be 
expected from the above summary, water exerts a much greater 
influence upon the form and structure of the plant than any other 
factor. 

9. The nature of water stimuli. A terrestrial plant is con- 
stantly subjected to the simultaneous action of water stimuli, the 
water content of the soil acting upon the roots, and the humidity 
of the air upon the leaves. Water content regulates the water 
supply, humidity the water loss. The two are compensatory, and 

7 



8 PLANT PHYSIOLOGY AND ECOLOGY 

the final response to a stimulus of either sort can only be deter- 
mined by reference to the action of the other. An increase or 
decrease in water loss produces a corresponding change in the 
amount of water absorbed, and a change in water supply tends to 
produce a consequent change in water loss. This is strictly true 
only when the stimuli are normal. For example, a decrease in 
humidity causes increased water loss, which is compensated, as a 
rule, by increased activity at the root surface. Frequently the 
water supply is insufficient to compensate for heavy or rapid water 
loss, and the proper balance can be reached only by closing the 
stomata. In the case of excessive supply or loss, neither compen- 
sation suffices, and the plant dies. A change of structure, i.e., 
adaptation to water stimuli, results when the compensation of 
supply for loss or the reverse is more or less inadequate, but not 
to a degree sufficient to cause death. In addition to this funda- 
mental compensating action of water stimuli upon the plant as a 
whole, water content affects the growth of roots in such a way 
that the direction of growth is determined by the distribution of 
the moisture of the soil. The rule is that roots turn and grow 
toward the area of greatest moisture. This phenomenon is known 
as hydrotropism: it will be discussed under absorption. 

10. Water content. The water content of a habitat is the 
total amount of water found in the layer of soil occupied by the 
roots. The water of lower strata may be raised, and ultimately 
used by the plant, but it is not properly water content until it 
reaches the roots. The water is found in the form of thin films 
surrounding the soil particles. The amount depends upon the 
thickness of these films. In soils that are saturated the films run 
together, forming drops and masses of water. In air-dry soils 
there is still a very thin film about the smallest particles. The 
amount of water content varies most widely in different habitats. 
Impervious rocks contain practically none, until cracks and rifts 
are formed by weathering. Marshes, ponds, streams, etc., repre- 
sent habitats with maximum water. Dry habitats, such as prairies, 
plains, gravel slides, sand-hills, etc., have a low water content, 
varying as a rule from 3-15%. Wet habitats vary from saturation, 
as in stream banks, wet meadows, bog hummocks, etc., where the 
percentage ranges from 20-80% in accordance with the soil, to the 
maximum found in bodies of water. Moist habitats, meadows, 
forests, and cultivated fields usually contain from 15-30%. 



THE WATER OF THE HABITAT 9 

ii. Influence of other factors upon water content. The most 
important differences between habitats are due to differences of 
water content. The latter arise largely from the effect of the other 
factors of the habitat. All of these have an influence either direct 
or indirect upon the amount of water present, but soil, rainfall, 
physiography, and humidity are the most important. A sandy 
soil contains less water than a clay soil, even when both receive the 
same amount of rain. It goes without saying that a habitat in a 
desert region has a lower water content than one in a humid region, 
because of differences in precipitation and humidity. Two habi- 
tats with the same soil and atmospheric conditions may owe their 
difference to slope, which tends to decrease water content. The 
latter is likewise affected by cover, which prevents rainfall from 
running away before it can be absorbed, and also decreases the 
loss by evaporation from the surface of the soil. Heat and wind 
decrease water content indirectly by decreasing the humidity, par- 
ticularly upon exposed slopes. The plant covering itself has the 
effect of a dead cover in reducing evaporation from the soil, but on 
the other hand it tends to decrease water content, owing to the 
use of water by the plants. 

12. Available and non-available water content. If a rooted 
plant is allowed to wilt and die, a careful examination of the soil 
shows that some water still remains. The amount depends upon 
the kind of soil, but all soils agree in the ability to retain some 
portion of the water content. This is due to the fact that the 
attraction of soil particles for the w r ater films increases as the film 
grows thinner, until finally it is greater than the attraction exerted 
by the root-hairs. At this point the plant is unable to obtain 
water, and it rapidly dies by wilting. The water thus held by the 
soil can not be used by the plant, and it is hence called the non- 
available w r ater, or echard. It is usually but a small part of the 
water commonly present, particularly in moist or saturated soils. 
In a fine-grained compact soil, like clay, the amount of non-avail- 
able water is large; in coarse-grained soils, e.g., sand and gravel, 
it is very small. Of the total water content, or holard, the larger 
portion can be absorbed by the plant, and is consequently termed 
available w T ater. The response of the plant to water content is 
determined by the amount available for absorption and not by the 
total amount present. This available w r ater, or chresard, differs 
for the different soils, and for dissimilar species of plants. It is 



10 PLANT PHYSIOLOGY AND ECOLOGY 

diminished by the presence of excessive amounts of salts in the soil, 
and also by unusually low temperatures. Hence in measuring the 
water content of habitats the best practice is to determine both 
holard and chresard, bearing in mind that it is the latter alone that 
results in adjustment and adaptation. 

13. Soil samples. In obtaining samples of soil for finding the 
water content, the usual practice is to remove the air-dried sur- 



Fig. 1. — Geotomes and soil can, showing at the left the plunger for remov- 
ing the soil core. 

face, noting its depth, and then to sink the soil-cutter or geotome, 
with a slow, boring movement, in order to avoid packing the soil. 
This tendency is further reduced by cutting a long core a decimeter 
at a time. As soon as the sample is dug, the plunger is used to 
press the core from the geotome directly into an air-tight soil can. 
The lid is screwed on as quickly as possible, and the number recorded 
immediatelv with such notes as are desirable. The cans should 



THE WATER OF THE HABITAT 11 

be numbered with paint on both lid and side in such a way that 
the number may be read at a glance. The rule is to weigh the 
cans as quickly as possible after the sample is taken, though when 
necessary they can be kept for several days without appreciable 
error. For weighing, delicate balances are to be preferred, but 
when these are not available, coarser balances which weigh accu- 
rately to one centigram give satisfactory results. The best method 
is to weigh the soil sample in the can. Turning the soil out upon 
the pan or upon paper saves one weighing, but there is always some 
slight loss, and the chances of serious mishap are many. After 
weighing, the sample is dried in a water-bath or oven. At a tem- 
perature of 100° C. this is ordinarily done in twenty-four hours; 
stiff clays require a longer time. High temperatures must be 
avoided with soils that contain much leaf-mold or other organic 
matter, in order that this may not be volatilized, and hence lead to 
an error in the result. When a drying-oven is not accessible, soil 
samples are dried in the air, preferably in sunshine. This usually 
takes several days, and a test weighing is generally necessary to 
determine that the drying-out is completed. The weighing of the 
dried soil is made as before. The can is carefully brushed out and 
weighed also. 

14. Computation of water content. To find the percent of 
water content, the second weight, i.e., of the dried sample and can, 
w', is subtracted from the first weight, w, of the original sample 
and can. The weight of the can, w 2 , is taken from the dried weight, 
w' '. The first result is then divided by the second, and the result 
is the percent of total water content, i.e., holard, figured upon the 

w — w' 

dry soil as a basis. The formula is— : ~ = W, in which W is the 

w —vr 

holard in percent. Water content has generally been computed 
upon the moist soil as a basis. 1 This method leads to inexactness 
in the comparison of habitats, however, and should be abandoned. 
Since most of the results so far obtained have been made in this 
way, it is necessary at present that the basis be taken into account 
in comparing the results of different workers. The most satisfac- 
tory method for the present is to express the results in grams per 
hundred grams of moist soil. For example, 20/100 indicates a 
water content that is 20% of the moist weight or 25% of the dry 
weight. 

1 Research Methods, 28. 



12 PLANT PHYSIOLOGY AND ECOLOGY 

15. Time of water content readings. Since conditions in a 
plant-house are nearly constant, a single reading of water con- 
tent made at any time is fairly representative. This is not the 
case in the field, owing to the influence of rainfall, evaporation, 
and gravity in changing the amount of soil water. An isolated 
reading has very slight value, and it is therefore necessary to 
obtain a basis for comparison by making either a series of read- 
ings in one place at different times or in different places at the 
same time. Whenever the proper amount of time can be given 
to field work, the best method is to select a series of stations or 
habitats, and to take readings throughout the entire series at 
different times. Under ordinary conditions the time of day at 
which a particular sample is taken is of little importance, since 
the variation during a day is usually slight. This does not apply 
to exposed wet soils or to soils which have just been wetted by 
rain. When a series of readings is made in different places, how- 
ever, it is better that the readings be made in rapid succession. 
Simultaneous readings are necessary only when it is desired to 
determine whether there is a difference in the rate of loss in the 
various habitats. 

16. Location of readings. In determining the location of 
readings in the field, it is desirable to obtain as great a range as 
possible. Where the topography is much broken, as in mountain 
regions, a series of stations a mile long will include a number 
of different habitats. In general, fewer habitats are accessible, 
and it is then necessary to locate a station in each of the more 
or less diverse areas of each habitat. Grassland, woodland, and 
marsh show striking differences of water content as well as of 
other factors. A hilly prairie that drops through meadowland 
into forest offers an unusually good opportunity for a series of 
stations that will show the effect of soil, slope, cover, etc., upon 
the water content. On account of the small differences always 
present, each station should be definitely located where differ- 
ences of soil, slope, etc., are the most striking. For the sake 
of future readings, the exact location of each station is perma- 
nently marked and recorded. Successive readings are made as 
near to the preceding ones as possible, though new samples 
should not be taken too near the old holes. A difference of a 
few feet produces no appreciable error, if the station is uniform 
in character. 



THE WATER OF THE HABITAT 13 

17. Depth of samples. The general rule is that the depth 
of soil samples is determined by the distance to which the roots 
penetrate. The practice is to remove the air-dried surface in 
which no roots are found, and to take a sample to the proper 
depth. When the actively absorbing root surfaces are localized, 
as in deep-rooted plants, it is necessary to confine samples to 
the area in which absorption occurs. This is especially true 
when the water supply of a particular species of plant is to be 
determined, but, in the case of vegetation in general, roots pene- 
trate to such different depths that a sample which includes the 
greater part of the distance concerned is satisfactory. The usual 
depth of a sample is 3 decimeters. In shallow or wet soils, cores 
to a depth of 1 or 2 decimeters suffice, while in very dry soils, 
and in the case of shrubs and trees, deeper samples are required. 

Experiment 1. Measurement of water content. Take a 2-dm. 
sample from a dry soil and another from a moist soil; take one also 
from a sand and a loam. Weigh, dry, and compute percentages upon 
the basis of the dry soil. 

18. The determination of available water. The amount of 
water that a plant can absorb from the soil can be readily deter- 
mined only by finding the amount left when the plant wilts com- 
pletely. This determination can easily be made in the labora- 
tory or plant-house, though in the field it is attended with some 
difficulty. A thrifty plant growing in a pot of medium size is 
the best for the purpose. It is necessary that the pot be glazed 
or covered with sheet rubber in order to prevent too rapid drying 
of the soil. At the beginning of the experiment, three soil samples 
are taken in such a manner that they will indicate any variation 
in water content in different parts of the soil. The depth of the 
core is regulated by the size of the pot and the position of the 
roots. The holard is found in the usual way, and is expressed 
upon the basis of 100 grams of dry soil, e.g., 25/100. The aver- 
age of the three samples is taken as representative: this average 
is most readily arrived at by weighing and drying the samples 
in one can. The soil is then permitted to dry out slowly. Sud- 
den drouth often impairs the power of absorption, and the plant 
wilts even though considerable available water is present. The 
proper time to take the second reading is indicated by the thor- 
ough wilting of the leaves. It is undesirable to wait for com- 



14 



PLANT PHYSIOLOGY AND ECOLOGY 



plete wilting, since the younger parts are able to draw upon the 
watery tissues of stem and root for some time after the plant 
is unable to obtain water from the soil. Three samples are again 
taken from the soil, and the average water content determined 
as before. This is the non-available water or echard. This is 
likewise computed for 100 grams of dry soil, and the result is 
subtracted from the holard. The final result is the available 
water, or chresard, expressed in the number of grams to 100 grams 
of dry soil. 

19. Chresard of habitats. In order to find the amount of 
water available for a plant in its own habitat, it is necessary to 
produce wilting by cutting off the water-supply. This is accom- 
plished by digging up a plant in its own soil and transferring 
it to a pot of good size. The pot is placed in the hole from which 
the plant is taken, and a canvas awning arranged to prevent 
wetting by the rain. Where the distance of the habitat makes 
this method difficult or impracticable, the plant is transferred 
to the plant-house. In either case the soil samples are taken 
as indicated above, and the chresard is arrived at in exactly the 
same way. The available and non-available water of six rep- 
resentative soils, together with the amount necessary for satu- 
ration, are indicated in the following table. The figures serve 
equally well to indicate percentages and the number of grams of 
water for each 100 grams of dry soil. It must be clearly recog- 
nized that these figures will not be exactly the same for every 
kind of sand, clay, etc.: 



Soil 


Holard 


Echard 


Chresard 


Sand 


14.3 
47.4 
59.3 
64.1 
65.3 
68.5 


.3 
9.3 
10.1 
10.9 
11.9 
16.2 


14 


Clay 


38.1 


Loess 


49.2 


Loam 


53.2 


Humus 


53.4 


Saline 


52.3 







Experiment 2. Determination of available water. Germinate sun- 
flower seeds in sand and in rich garden soil. Transplant the seedlings 
from time to time so that they will be from 1 to 2 feet in height when 
they reach a 6-inch pot. Find the holard and echard in the way in- 
dicated, and from these compute the available water content of each 
soil. 



THE WATER OF THE HABITAT 



15 



MODIFYING FACTORS 

20. Influence of soil upon water content. The soil of a habitat 
is of the greatest importance in determining the amount of water 

content, and also the kind of water, i.e., the chemical substances 
found in solution. The amount of water present is directly de- 
pendent upon the texture or fineness of the soil, that is, upon 
its physical properties. The kind and amount of nutrient mate- 
rial dissolved in the water are determined by the chemical nature 
of the soil. In studying the influence of the latter, it is conse- 




FiG. 2. — Glacial boulders at Lake Moraine, below Pike's Peak, in which 
the disintegrating action of weather is aided by the roots of trees. 

quently necessary to examine the physical structure and to deter- 
mine the chemical composition. These are not of equal value, 
however. The amounts and kinds of soluble materials in all ordi- 
nary habitats are so nearly alike that differences in chemical com- 
position are of little importance. They play a large part only 
where soluble chemical compounds are present in excessive amounts, 
as in alkaline soils, or when the amount of humus is unusually 
large or deficient. An excess of soluble salts hinders absorption 



16 



PLANT PHYSIOLOGY AND ECOLOGY 



and reduces the chresard, while an excess of acids has the oppo- 
site effect. The structure of the soil, on the contrary, has an 
almost absolute control upon the fate of the water that enters 
the ground, in addition to its influence upon the water that runs 
off. It determines the amount of water drained away in response 
to gravity, and also the amount that can be raised from the lower 
layers by means of capillary action. The total water content is 
dependent in the first place upon the amount of water that soaks 
into the soil. Of this, the holard is that part which the soil holds 
in spite of the action of gravity, together with that which may 
be raised from time to time by capillarity. The chresard, how- 
ever, is only that part which the root-hairs are able to take up 
in opposition to the pull of the soil particles. 

21. Origin and structure. Soils are formed from rock by 
the action of weathering. The latter is due to the influence of 




Fig. 3. — Decomposition of a granite boulder into gravel and sand, and 
the further breaking down of these by the roots of herbaceous plants. 



both physical and biological factors, acting separately or together. 
Weathering consists of two processes. The one is disintegration, 
by which the rock is broken into fragments of various sizes; the 



THE WATER OF Till! HABITAT 



17 



other is decomposition, by means of which the original rock, or 
its fragments, is broken down into minute particles. These two 
processes usually go hand in hand, although as a rule one is more 
marked than the other. Their relative importance is deter- 
mined by the character of the rock, and by the forces that act 
upon it. Hard rocks, e.g. granite, as a rule, disintegrate more 
rapidly than they decompose, while sedimentary rocks, such as 
sandstone, limestone, etc., tend to decompose more rapidly than 
they disintegrate. In many cases of weathering, the two pro- 
- are of equal importance. 
22. Water capacity. A soil ow T es its capacity for water to 
the fineness of its particles. Since the water is in the form of 




Fig. 4. — Diagrammatic sketch showing the structure of a fine soil and the 
position of root -hairs in it. The root epidermis (e) gives rise to root- 
hairs (h) which push their way between the angular soil particles sur- 
rounded by thin water films. The air spaces are white. (After Sachs.) 



thin films upon the soil particles, the amount necessarily increases 
wdth an increase in the water-holding surface. The latter is in- 
creased as the particles become finer and more numerous, and 
thus produce a greater aggregate surface. The -movement of 
water upward and dowmward in the soil is likewise dependent 
upon the size of the particles. As the latter become finer, the 



18 PLANT PHYSIOLOGY AND ECOLOGY 

irregular capillary spaces between them grow smaller, and the 
upward or capillary movement is increased. On the contrary, 
the downward movement of water, i.e., percolation, which is 
caused by gravity, is retarded by a decreasee in the size of the 
soil grains, and hastened by an increase. The properties of the 
soil which regulate the upward and downward movement of 
water are respectively capillarity and porosity. Both are de- 
pendent upon the structure or fineness of the soil, though in a 
manner directly opposite to each other. Capillarity increases 
with the fineness of the soil, porosity with its coarseness. Capil- 
larity augments the water content of the upper layers, while 
porosity decreases it. Upon this basis alone, soils fall into two 
groups, capillary soils and porous soils, the former fine-grained 
and of high water content, the latter coarse-grained and with 
relatively little water. However, a third factor of great impor- 
tance must be taken into account. This is the pull exerted upon 
each water film by the soil particle itself. The pull seems to in- 
crease in strength as the film grows thinner, and this explains 
why it finally becomes impossible for the root-hairs to draw mois- 
ture from the soil. This property, like capillarity, is most pro- 
nounced in fine-grained soils, such as clays, and is least evident 
in the coarser sands and gravels. It furnishes the explanation 
of non-available water, and indicates that the chresard is directly 
connected with soil texture. 

23. Chemical nature of soils. Apart from the effect of exces- 
sive amounts of acids or salts, the chemical nature of the soil 
is of slight importance, except in the case of soils exhausted by 
intensive cultivation. In nature the necessary nutrient salts 
are so uniformly distributed that the chemical composition of 
the original rock is immaterial. A soil can modify the plants 
upon it only through its water content, or the soluble salts, or 
solutes, that it contains. Hence, when differences of structure 
or distribution occur between habitats with different soils, the 
cause is not to be sought in the fact that the soil is silicious, cal- 
careous, or argillaceous, but in the effect of the texture upon water 
content. It now appears entirely incorrect to ascribe the pres- 
ence or absence of certain species on limestone soils to the chem- 
ical nature of the latter. The most important chemical elements 
in the soil appearing in the form of salts and connected with the 
growth of green plants are nitrogen, sulphur, phosphorus, iron, 



THE WATER OF THE HABITAT 19 

potassium, calcium, ami magnesium. These occur usually as 
nitrates, sulphates, phosphates, chloride's, carbonates, or oxides. 

The effect of alkalies and acids in the soil upon water con- 
tent and absorption is not altogether understood. Experiments 
indicate that alkalies hinder absorption, and acids promote it. 
In other words, alkalies reduce the amount of available water, 
while acids increase it. Alkaline soils are really dry soils, a fact 
clearly established by the character of the plants that grow upon 
them. On the contrary, acid soils are wet; usually indeed they 
show maximum water content. They contain plants which are 
adjusted to an excessive water supply. The majority of such 
plants exhibit adaptations to water, but some of them have the 
appearance of dry land plants, or xerophytes. A careful exam- 
ination of the structure of the latter reveals modifications due to 
water. Consequently, it seems almost certain that such "bog 
xerophytes" are dry hand plants, which in coming to grow in 
water have retained certain superficial ear-marks of the original 
habitats. 

24. Air content. In all soils, but particularly in acid ones, the 
air content is a factor of considerable importance, owing to the 
constant use of oxygen by the roots. The amount of air present 
depends upon the water content and the compactness of the soil. 
Air content varies inversely as the w T ater content: it is large in dry 
soils and very small in wet ones, especially those covered with 
water. Water plants, i.e., hydrophytes, show characteristic modi- 
fications called forth in response to a low air content. When a 
soil becomes packed, the movement of the air into and through the 
soil is impeded, and a very important task of cultivation is to keep 
the soil so stirred that the air content does not fall too low. " Sour " 
soils, including "sour " bogs, owe their nature to the production of 
organic acids in the presence of a low supply of oxygen. When 
stirring the soil is possible, "sourness " is easily remedied, since 
complete decomposition prevents the formation of acids. 

25. Determination of soil properties. Since the soil is prima- 
rily important because of its influence upon water content, the 
determination of soil texture is the principal task in this connection. 
In an elementary study of habitats it is sufficient to know that a 
soil, or the soil water, is acid or alkaline, without further reference 
to amount. Such a test is readily made in the field by means of 
fresh strips of litmus paper. Air content is in large degree a result 



20 PLANT PHYSIOLOGY AND ECOLOGY 

of water content, and has but an indirect or obscure effect upon 
the water present. The texture of the soil is chiefly a matter of 
size of grains or fineness. The latter can best be ascertained by the 
use of sieves. Two sieves of 1 mm. and of .05 mm. mesh will make 
it possible to separate the soil into gravel, sand, and silt-clay. The 
structure of the soil is then expressed in percents, e.g., gravel 10%, 
sand 15%, silt-clay 75%). 

A mechanical analysis of a soil throws little additional light 
upon its behavior with respect to water. It is much more helpful 
to know its porosity and capillarity under conditions as nearly 
natural as possible. Both of these are determined by using a 
cylinder of the soil concerned and noting the rate with which water 
moves downward or upward through it. For general purposes, 
however, a measurement of porosity suffices for both, since capil- 
larity varies inversely as the porosity. Thus sand is extremely 
porous, but possesses very little capillarity. Clay, at the other 
extreme, exhibits strong capillary movement, but is very slightly 
porous, while intermediate soils approach the one or the other in 
behavior in direct proportion to the amount of the predominant 
constituent. The use of soil-cores from typical habitats yields the 
most satisfactory data concerning porosity and capillarity in rela- 
tion to actual water content. When these are not obtainable, 
instructive results can be obtained by using loose soils in the plant- 
house, provided that the soils are well packed before the experi- 
ments are made. The presence of decaying vegetable matter 
increases the porosity of a soil, and correspondingly decreases its 
capillarity, but after complete decomposition humus tends to 
increase capillary action, especially in coarse soils. The amount of 
humus may be readily ascertained by weighing a soil before and 
after it is thoroughly burned. 

Experiment 3. Porosity and rate of evaporation. Fill three 2-inch 
flower-pots respectively with sand, loam, and clay that are nearly dry 
but not powdery. Pack each soil firmly until its surface is a half-inch 
below the edge of the pot. Place each pot in a tall Stender dish, and 
slowly pour water upon the clay until the former runs into the dish 
through the hole in the pot. Pour the same amount upon both sand 
and loam, and measure the amount of water that percolates through 
each. Weigh each pot of soil, and, together with a graduate of water 
having an equal surface, put them in a sunny place. Weigh each every 
day or two for a week or more. As soon as the sand shows no further 



Tin: WATEB OF THE HABITAT 21 

loss, place the three soils in a water-bath and dry them oul completely. 
Make a final weighing. Compute the percentage of water in the satu- 
rated soils at the beginning, the rate of loss from each as compared 
with the water surface, and the amount of water in each at the time 
the sand showed no further loss in the air. 

26. Influence of precipitation upon water content. In all 
habitats except those where the supply of water is constant, owing 
to the presence of springs, streams, ponds, or other bodies of water, 
the dependence of water content upon rainfall is absolute. Soil 
and slope determine how much of the latter finds its way into the 
ground, but their action is secondary. Daily rains are able to keep 
practically any soil saturated, regardless of its character or the 
slope. All habitats not covered with water reach their maximum 
water content immediately after a heavy rain or during the rainy 
season. The water decreases gradually throughout a dry period 
or season, only to again approach the maximum when precipitation 
takes place. The latter occurs in various forms, such as rain, 
hail, dew, frost, and snow. Of all these, rain is by far the most 
important. In spite of local exceptions to the rule, hail is too 
infrequent to be taken into account. Frosts have at best only 
a slight and fleeting effect upon water content, especially in view 
of the fact that they usually fall outside the growing season. Snow- 
fall is often of great importance. It not only acts as a cover to 
prevent evaporation, but upon thawing it also enters the soil 
directly just as rain does. The loss by run-off from slopes is much 
greater, owing to the frozen condition of the ground. The exact 
importance of dew is not easily determined. Dew r is almost always 
too small in amount and too fleeting to add directly to the water 
content of the soil. By its own evaporation it doubtless decreases 
in a slight degree the amount of water lost by the soil and by 
bedewed plants. In studying the w r ater content of habitats a 
knowledge of the amount of rain usually suffices, though in the 
study of habitats in spring, the amount and distribution of snows 
must also be taken into account. 

27. Measurement of rainfall. Rainfall is measured by means 
of a rain-gauge, an instrument which collects in a narrow vessel the 
rain falling upon a large surface. In the standard instrument the 
ratio of surface between receiver and tube is 10 to 1. A direct 
measurement of the water in the tube must be divided by 10 to 
give the rainfall, or a standard measuring-rod, upon which this 



22 



PLANT PHYSIOLOGY AND ECOLOGY 



compensation is already made, may be used. In elementary work 
it is impracticable to measure the rainfall in different habitats. It 
is fairly satisfactory to use the reports of rainfall obtained from a 
neighboring weather station when the latter is not more than a 
few miles distant. The effect of rainfall upon water content is best 
ascertained by taking soil samples in different habitats immediately 
after a rain, and then determining the increase in water content. 
In open, hilly regions there sometimes occur in spring differences 
in water content which can only be explained by a knowledge of 
the distribution of the late snows of winter. 

28. Physiography. Physiographic factors are altitude, expos- 
ure, slope, and surface. There are in addition certain physio- 




Fig. 5. — Mountain sides showing differences of slope, exposure, and cover. 

graphic processes, such as weathering, erosion, and sedimen- 
tation, which play a fundamental part in changing habitats, but 
these do not act directly upon water content. The latter is di- 
rectly influenced by slope and surface, while altitude and exposure 
operate only through humidity. Cover, either dead or living, while 
not exactly a physiographic feature, affects water content in much 
the manner of surface, under which it may well be considered. 



THE WATER OF THE HABITAT 23 

29. The influence of slope. By slope is meant the inclination 

of the surface of a habit at with respect to the horizon. The 
principal effect of slope is in controlling run-off and drainage, 
aiul through them water content, although they are at the same 
time affected by surface and soil texture. Slope has also a less 
direct influence through its action upon heat and wind, which 
in turn modify humidity and thus affect the water content. Slope 
is expressed in degrees of the angle made by the intersection of 
a line bounding the surface and the base line. It is measured 
by means of a clinometer, a simple instrument in which a line 
and plummet indicate the angle of slope upon a semicircle 
graduated in degrees. In making a reading, it is desirable to 
use a basing strip, a piece of wood 1 meter long and 5 centimeters 
wide, with a true edge. The basing strip is placed upon an area 
typical of the slope, and is pressed down firmly to equalize irregu- 
larities. The clinometer is moved gently along the upper edge, 
causing the marker to swing freely. When the latter comes to 
rest, the instrument is carefully turned upon its back, and the 
angle of slope read directly in degrees. Two or three readings 
in different parts give a very satisfactory mean for the entire 
habitat. The angle of slope can not be directly connected with 
the amount of water content, because of the other factors con- 
cerned. The rule is that the rainfall lost by run-off increases 
with the angle, and the w T ater absorbed correspondingly decreases. 
In two or more areas essentially alike in soil, cover, and rainfall, 
differences in water content are directly determined by differ- 
ences in slope. 

30. The influence of surface. The surface of a habitat often 
shows irregularities which retard the movement of run-off and 
cause more of the rainfall to soak into the soil. The soil itself 
often shows such irregularities, e.g., the rocks of boulder and 
rock fields, the hummocks of meadow r s and bogs, the mounds 
of prairie-dog towns, the raised tufts of prairies and sand-hills, 
the minute gullies and ridges due to erosion, etc. The influence 
of these is usually not great, but it is always appreciable, and 
in some cases of considerable importance. Their effects are often 
measurable by means of soil samples, but it is impossible to ex- 
press the character of the surface in definite terms. It must 
suffice to describe the surface as even or uneven, and to indi- 
cate the kind and amount of unevenness. 



24 PLANT PHYSIOLOGY AND ECOLOGY 

The same is true of cover, which is usually of far greater and 
more universal importance. Dead and living cover retard run- 
off and reduce the amount of water lost from the surface of the 
soil. By their decay, plants add humus to the soil, thereby in- 
creasing its capacity for absorbing and retaining water. Dead 
cover is of less importance, since it is found chiefly during the 
resting period alone, but it plays a part of some value by catch- 
ing and holding drifting snow. The cover of living vegetation 
reacts upon the habitat in a much more vital fashion. While 
it has a powerful effect in increasing water content, it reduces 
it also by reason of the water lost through evaporation from the 
plants. Cover can be expressed only in general terms of density 
and height at present, and it suffices, as a rule, to indicate the 
character of the plant covering. In this connection it should 
be noted that other biological factors, viz., man and other ani- 
mals, often exert an influence upon water content. Except in 
special cases, such as irrigation and drainage, this effect is exerted 
through other factors, and does not need further consideration 
here. 

31. The influence of climatic factors. All the atmospheric 
or climatic factors of a habitat have an effect upon water con- 
tent, either immediate or otherwise. Such factors are humidity, 
temperature, wind, pressure, and light. The influence of the 
last is slight and very indirect. Temperature, wind, and pressure 
can change the amount of water in the soil only through humidity, 
and hence they will be further considered under the latter. 
Humidity acts upon the plant and upon soil water in the same 
way, i.e., by controlling evaporation. It affects water content 
directly through water loss from the soil, and indirectly inas- 
much as the water lost by the plant is first drawn from the soil. 
Since it is a direct factor, it will be more fully considered in the 
following sections. It is mentioned here merely to complete 
the list of factors that play a larger or smaller part in modifying 
or controlling the water content of the habitat. 

32. Humidity. The moisture of the air, which is in the form 
of vapor, is termed humidity. Owing to the nature of the medium 
in which it occurs, humidity is much more uniformly distributed 
than the water content of the soil. For the same reason it fluctu- 
ates to a much greater degree. It differs from water content 
also in that a part of the latter, i.e., the echard, is always non- 



THE WATER OF THE HA1UTAT 



25 



available for the purposes of the plant, while the whole humidity 
of the air is the stimulus that controls the water loss of a leaf. 
The actual amount of water present in the air is called the abso- 
lute humidity, and is expressed in milligrams per cubic centi- 
meter. The relative humidity is the relation between the amount 
of moisture in the air and the amount necessary to saturate the 
air under given conditions. It is expressed in percents, and is 
in common use as the expression of humidity. 

33. Modifying influence of temperature and wind. Humidity 
is affected by temperature, wind, pressure, altitude, exposure, 




Fig. 6. — Dwarf spruces at timber line on Pike's Peak, produced by the 
drying and the mechanical action of almost constant winds. 



cover, and water content. High temperatures increase the capacity 
of the air for moisture and low temperatures diminish it; the 
former lower the percent of relative humidity, the latter raise it. 
Of two regions, or two habitats with the same rainfall, the warmer 
is the drier. During the day the relative humidity falls as the 
temperature rises, and rises in the evening as the air grows cooler. 
Wind has also a powerful effect upon humidity in that dry winds 
lower the amount of air moisture by mixture or removal, while 



26 



PLANT PHYSIOLOGY AND ECOLOGY 



moist winds exert an opposite influence. The most important 
action of wind is the removal of the more humid air ordinarily 
occurring above the plants of a habitat, and its replacement by a 
drier air. This has the effect of keeping the immediate humidity 
low. This may be readily verified by taking readings of humidity 
in a sheltered area and in one exposed to the full effect of a strong 
wind. The moist winds that blow across a large body of water or 
those that precede a rain either do not have this effect or possess it 
in but a slight degree. The exact influence of wind upon humidity 
is best shown in a hilly habitat, such as an undulating prairie. If 
the velocity of the wind is determined by means of a hand ane- 
mometer for ravine, slope, and ridge, and simultaneous readings of 
humidity are taken, the relative humidity will be found to de- 
crease with the increase in wind velocity. 

34. Influence of pressure and physiographic factors. Pressure 
modifies humidity by varying the density of the air, and hence its 




Fig. 7. — Station on the summit of Mount Garfield (3800 m.) for deter- 
mining the effects of altitude by means of plants and instruments. 



power to hold moisture. The daily fluctuations which constitute 
weather are slight and are of little importance, except in their rela- 
tion to rainfall. The effect upon humidity is much more pronounced 
when differences in altitude bring about permanent differences in 



THE WATER OF THE HABITAT 27 

pressure. Altitude has been thought to influence vegetation chiefly 
by virtue of increased light and decreased heat. Recent studies of 
the author seem to prove conclusively that light is of practically 
no importance, and that the marked dwarfing of alpine plants is 
due largely to the great rarefaction of air by which evaporation is 
increased. Exposure, i.e., the position of a slope with respect to 
the sun affects humidity through the action of sun and wind. 
Slopes longest exposed to the sun's rays receive the most heat; 
consequently slopes with a southern exposure regularly show some- 
what lower humidities than those with northern exposures. The 
effect of wind is most pronounced upon those slopes exposed to 
prevailing dry winds. As a rule, these are southern or southwestern, 
and for reasons both of temperature and wind these are usually the 
driest slopes of hills and mountains. Cover increases humidity by 
reducing the influence of temperature and wind. In addition a 
living cover supplies moisture to the air in consequence of evapora- 
tion from the plants that compose it. A similar effect is produced 
by the water content of moist soils, particularly in forests and 
thickets where the air is sheltered from sun and wind. 

35. Effect of climate and habitat. The general humidity of 
a habitat depends upon climate and location with respect to bodies 
of water. In comparison with each other forested regions show 
high humidities, while deserts have low humidities. Coast regions 
are moist, inland regions relatively dry, lowlands are more humid, 
table-lands and mountains less humid as a rule. In a particular 
habitat the relative humidity approaches or reaches saturation 
during rain or fog, and then gradually decreases to a minimum 
just before the next rain-storm. There is also a daily maximum 
and minimum. The highest relative humidity, except when dis- 
turbed by rain, usually falls at 3 or 4 a.m. It decreases slowly until 
7 or 8 a.m. and then falls more rapidly to a minimum at about 4 p.m., 
from which point it rises slowly to the maximum. Variations within 
the habitat arise chiefly through differences in protection from sun 
and wind. For somewhat similar reasons the relative humidity is 
greatest just above the surface of the soil; it is less at the level of 
the vegetation, and still less a meter or more above the latter. 

36. Measurement of humidity. Humidity is measured by 
means of a psychrometer. Of the latter there are three types: 
the sling, the cog, and the stationary psychrometer. All consist 
of a wet-bulb and a drv-bulb thermometer set in a case. The first 



28 



PLANT PHYSIOLOGY AND ECOLOGY 



two are designed to be moved or whirled in the air. The same 
principle is applied in each, viz., that evaporation produces a 
decrease in temperature proportional to the amount of moisture in 
the air. The dry-bulb thermometer is an ordinary thermometer, 
while the wet-bulb one is covered with a cloth that can be moistened. 
The former indicates the normal temperature of the air, the latter 
gives the reduced temperature due to evaporation. The relative 
humidity of the air is ascertained by means of the proper tables, 
from two terms, i.e., the air temperature and the amount of reduc- 
tion shown by the wet bulb. 

37. Sling and cog psychrometers. For field work the sling 
and cog psychrometers are much more convenient than the station- 




Fig. 8. 



-The cog psychrometer. The wet bulb is the one covered 
with cloth. 



ary form. They are generally considered to be more accurate also, 
since the movement prevents the accumulation of moisture about 
the wet bulb. Of the two, the cog psychrometer is the more con- 



THE WATEB OF THE HABITAT 29 

venient and satisfactory. It is smaller, more compact, and the 
danger of breakage in use or in carriage is extremely small. It has 
the further advantage of making it possible to take readings in a 
layer of air less than 3 centimeters in thickness, and in any posi- 
tion. The use of the sling psychrometer is attended with grave 
danger to the instrument in a free space less than two yards across. 
The cog psychrometer has a single disadvantage owing to the neces- 
sary use of short thermometers. To secure the proper range, Centi- 
grade thermometers must be used, and the readings thus obtained 
must be converted into Fahrenheit temperatures before the humid- 
ity can be determined from the usual tables. 1 

38. Making a reading. In general, observations should be 
taken facing the wind. It is also a wise precaution to shift the 
position of the instrument a foot or more during the reading, except 
when the humidity of a definite layer is desired. The cloth of the 
wet bulb is first moistened with water carried in a small 50-cc. 
bottle for this purpose. Distilled water is preferable, but tap- 
water and the water of streams may be used without appreciable 
error, if the cloth about the wet bulb is changed occasionally to 
prevent the accumulation of dissolved material. The water is 
poured slowly upon the cloth of the bulb until it is completely 
wetted, care being taken not to wet the dry bulb. As the cloth 
absorbs water reluctantly when perfectly dry, a pipette or a brush 
is usually a valuable aid in wetting it quickly. The temperature 
of the water used is of slight consequence, though readings can be 
made more quickly when the temperature is not too far from that 
of the air. The psychrometer is held in the proper position, i.e., 
the bulbs are placed in the layer of air to be studied, unless a gen- 
eral reading is to be made, and are then rotated at an even rate and 
at a moderate rapidity. As the reading must be made when the 
mercury of the wet bulb reaches the lowest point, the instrument 
is usually stopped after 100 revolutions, and the position of the 
column is noted. The lowest point is often indicated by the ten- 
dency of the mercury to remain stationary. As a rule, the lowest 
point can be known with certainty only when the next glance shows 
a rise in the column. Check readings of this nature must be made 
every 25 or 50 revolutions in order to make sure that the mercury 
has not reached the minimum and then begun to rise while the 
instrument is in motion. In noting the final reading, care must be 
1 Research Methods, 39. 



30 PLANT PHYSIOLOGY AND ECOLOGY 

taken to secure it before the mercury begins to rise in consequence 
of stopping the movement. For this reason it is desirable to shade 
the psychrometer with the body when looking at it in the sunshine, 
and to take pains not to breathe upon the bulbs or to bring them 
too near the body. At the moment when the wet bulb registers 
the lowest point, the dry bulb should also be read and the results 
recorded. 

39. Use of humidity tables. To ascertain the relative humid- 
ity, the difference between the wet- and dry-bulb figures is obtained. 
This difference, together with the dry-bulb temperature, is referred 
to the tables. A variation in temperature has less effect than a 
variation in the difference. In consequence, the dry-bulb reading 
is expressed in the nearest unit, and the difference is reckoned to 
the nearest .5. Since the humidity varies with the air pressure, 
it is necessary to use the table computed for the normal barometric 
pressure of the place under consideration. Humidity tables are 
usually computed for pressures of 30, 29, 27, 25, and 23 inches (76, 
73.5, 68.5, 63.5, 58.5 cm.). For mountain regions over 2100 meters 
(7000 feet) additional tables are desirable, but the table for 23 
inches will meet all ordinary requirements, since the effect of 
pressure is small within the usual range of growing-period tem- 
peratures. 

Experiment 4. Measuring humidity. Use a cog psychrometer to 
determine the range of humidity at 8 a.m., 12 m., and 4 p.m. Make 
readings in quick succession in the plant-house, and in sun and shade 
out-of-doors, and find the relative humidity for each. 

40. Method of habitat study. A real knowledge of physical 
factors, and of the habitats which are constituted by them, can be 
obtained only by the use of factor instruments in the field. Such 
knowledge is of the most fundamental importance in discovering 
the causes which control the functions and structures of plants, and 
their grouping into plant formations. All these objects can be 
obtained by establishing a series of stations, and using the members 
of the class to take simultaneous readings in them at different times 
of the year. The ideal method is to begin such a series just before 
the opening of spring, and to continue it at proper intervals through- 
out the entire growing period. This is scarcely feasible in the great 
majority of cases, and the most practicable method is to take a set 
or two of readings in the fall, and the same number in winter. Then, 



TIIK WATER OF THE HABITAT 31 

in the spring, readings should be taken every week or two until the 

work closes. 

41. Choice of stations. The stations of a series should be 
chosen with care, and as a result of considerable previous knowledge 
of the locality to be studied. The stations must not be too numer- 
ous nor too far apart. Within these restrictions, however, they 
should represent several distinct formations, and as many dissimi- 
lar areas in each as possible. A series can not well be more than 
two miles long, and one of a mile, or even a half mile, is to be pre- 
ferred. A good series will contain at least a dry, a wet, and a shadj r 
habitat, e.g.. a prairie, a swamp, and a forest. In class work of 
this sort at Lincoln, two series were first established, one in prairie, 
the other in woodland. For reasons of convenience and time- 
saving, these are now replaced by a prairie series consisting of the 
following stations: (1) meadow, (2) crest of ridge, (3) northeast 
slope, (4) ravine, (5) southwest slope, (6) sandy ridge, (7) willow 
thicket. (8) high prairie, (9) bog, (10) south slope. 

42. Constant factors. After the stations have been chosen, 
their location should be permanently indicated in such a way that 
they can be readily found from time to time. In order to avoid 
mistakes, the instructor should first take the entire class through 
all the stations, pointing out the general differences and illustrating 
the use of instruments not already familiar. If this does not take 
too Ions:, readings of the more constant factors, water content, 
slope, exposure, surface, and cover are made at the last station, and 
a student equipped with thermometer, psychrometer, and pho- 
tometer is left in charge. At each succeeding station the same 
plan is followed, so that upon reaching the first station the con- 
stant factors have all been read, and there is an observer at each 
station prepared to make readings of the variable factors. When 
the preliminary survey has occupied all the time available, the 
same method is employed upon the second visit, but the beginning 
is naturally made at the first station. The observers are shifted 
upon successive visits so that each student has an opportunity to 
become acquainted with every station. When the class is large, 
two or more students may be left at a station, and the work 
divided between them. 

43. Simultaneous readings. The task of obtaining readings 
at the same moment is met by taking observations upon signal. 
The instructor places himself at a commanding station, prefera- 



32 PLANT PHYSIOLOGY AND ECOLOGY 

bly near the middle of the series, and gives the signals by a shout 
or whistle at the proper interval. Considerable care and prac- 
tice are required in order to do the last satisfactorily. Suffi- 
cient time must be given for the operation of the instrument 
and the making of the record. In addition, a period which is 
long enough for each instrument to reach the proper reading 
must be permitted to elapse. For example, in a series which 
contains a gravel slide and a forest, a thermometer which has 



Fig. 9. — Observers making simultaneous readings of humidity in a series 
of stations in the prairie formation at Lincoln. 

just been used for an air reading will require four or five times 
as long an interval to respond to the temperature of the gravel 
as to that of the cool forest floor. In such a series, the place 
where the response is slowest or greatest often makes the best 
signal-station. The instructor records the exact time of each 
signal, and notes any general changes of sky or wind that pro- 
duce fluctuations at the time of reading. Temperature, humidity, 
and wind are read usually at li meters, and at the surface of 
the soil. Soil temperatures are obtained from the holes left in 
making soil samples. These holes are closed with corks to pre- 



THE WATER OF THE HABITAT 33 

vent a change in temperature, and are used only on the day they 
are made. Light readings are of course necessary only when 
one or more stations are shaded. For the sake of convenience 
and accuracy, factors are always determined in the same 
order, viz., temperature, humidity, wind, light; and the same 
is true of the various points or levels, e.g., 1J meters, surface. 
and soil. 

44. Point and hour readings. Readings taken as above at 
the same point in the stations of a series are said to be made at 
the same level, as the lj-meter level, the surface level, etc. 
Readings may also be taken simultaneously through the different 
points of a single station. In this work, the observers are grouped 
in each station in such fashion that they do not interfere with 
the correct reading of each instrument. Such determinations are 
most valuable in the case of temperature, which shows greater 
differences at the various levels. Important differences of 
humidity and wind are also discovered, and, in layered formations, 
marked variations in the amount of light. Series of this sort 
are likewise read upon signal. Hour series are indispensable for 
obtaining the variation of each factor during the day. They 
are read for each level upon signal in the manner already indi- 
cated, but the series is repeated every hour throughout the day. 
The number and position of the levels in all of the different series 
are properly determined by the character of the vegetation. In 
general, there should be levels corresponding to the surface, to 
the height of the herbaceous vegetation, and an air level above 
the latter. For temperature, one or two soil levels are necessary 
also. 

45. Records. A definite form of field record saves much 
time and prevents many mistakes. Printed blanks of the form 
indicated below, 7fx9| inches in size, have been found to be 
the most satisfactory. Each blank suffices for recording two 
full sets of readings through ten stations. The details may of 
course be modified as seems desirable. The blanks are carried in 
a cover protected with oil-cloth. The field readings are entered 
directly in ink in the case of temperature and wind, while light, 
humidity, and water content are recorded only when the final 
results are obtained, field memoranda being employed for the 
direct readings. 



34 



PLANT PHYSIOLOGY AND ECOLOGY 



Oak-hickory forest. 
April 20, 1901. Clear. Southeast wind. 





Temperature. 


Light. 


Humidity. 


Water content. 


Wind. 


Time, 






















I 


A.M. 


10:40 
Hm. 


10:46 
Surf. 


10:50 
1dm. 


10:55 
2 dm. 


11:00 
4 dm. 


12:00 
Hm. 


12:05 

Surf. 


11:10 
Hm. 


11:20 

Surf. 


Percent. 


11:30|11:45 


Sta. 


1dm. 


2 dm. 


4 dm. 


Hm. 


Surf. 


1 


16 


25 


9.6 


8.4 


7.8 


.08 


.06 


73 


81 


24.2 


19.2 


19.5 


298 





2 


16.2 


30.5 


8.5 


8.4 


7.8 


.11 


.09 


73 


86 


22 


22.5 


19.4 


375 


2 


3 


16.2 


17.8 


7.6 


7.8 


8 


.08 


.06 


73 


95 


22.1 


20.4 


21.6 


640 


6 


4 


15.6 


26.2 


10.6 


8.4 


8.2 


.06 


.03 


81 


95 


25.4 


23.1 


22.4 


275 


12 


5 


17.6 


25.4 


7.6 


7.4 


7.2 


.03 


.02 


90 


95 


27.2 


19.8 


18.8 


178 


2 


6 


16.2 


20.2 


8.4 


7 


6.2 


.02 


.01 


82 


90 


27.6 


20.8 


18.8 


115 


4 


7 


15.8 


17.2 


6.4 


6.4 


6.1 


.05 


.04 


82 


90 


23.8 


19 


19.3 


60 






46. Kinds of curves. The most graphic way of bringing out 
the factor differences between habitats or stations is by means 
of curves. The factors that lend themselves most readily to this 
method are the variable ones, water content, humidity, light, 
temperature, and wind. Curves representing these are spoken 
of respectively as water-content curves, humidity curves, etc. 
With regard to the time and position of the readings upon which 
they are based, they are divided into level, station, and point 
curves. A level curve is one based upon readings made at the 
same level through a series of stations, e.g., the level curve of 
surface temperature. The station curve shows the variation of a 
factor through the different points at which readings are made in 
a single station. The point curve has for its basis the hourly or 
daily variation of a factor at a single point in a station, such as the 
variation in humidity during the day at the surface of a barren 
ridge. All of these may be simple curves when based upon a single 
reading through a level, station or day, or mean curves when they 
are based upon the average of a number of such readings. 

47. Combinations of curves. Curves are often combined in 
order to permit of a ready comparison between them. Com- 
bination is brought about by tracing upon the same sheet the 
curves to be compared. Dissimilar curves, e.g., level and sta- 
tion, can not be combined. Colored inks are an absolute necessity 
in making combinations. The principle underlying their use 
is that curves which approach closely or cross each other must 
be traced in inks that contrast sharply. It is important to use 
the same color invariably for the same level or point. The variety 



Prairie: 4 2 


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- {I 
































































































































































36 










































































































































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r-38- 




























































































































































































































































































































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FIG. 10. — Combinations of water content curves, showing the holard in 
per cent at depths of 5. 10 and 15 inches through seven stations in 
the prairie formation. The two sets of readings were separated by an 
interval during which rain fell. 



THE WATER OF THE HABITAT 35 

of ways in which curves can be combined is almost endless. For 
the beginner, however, the most satisfactory are those in which 
but one factor is taken into account. The most useful are those 
in which the curves of temperature, or of any other factor, for 
the different levels are drawn together. Similarly, the curves 
showing the variations of temperature for each station may be 
combined. A combination of the greatest value is obtained by 
contrasting the curves of holard and chresard for a series of sta- 
tions. A very interesting combination may be obtained by 
arranging the series of curves for two distinct habitats, such as 
prairie and forest, side by side upon the same sheet, thus per- 
mitting the direct comparison of the curves for various factors. 

48. Plotting curves. The plottkig-paper employed is ruled 
in centimeter squares which are divided into 2-millimeter units. 
The sheet is 24x18 cm. in size, thus making it possible to file 
the curves in the record book. A fine-pointed pen, such as the 
Spencerian No. 1, is used for plotting. The inks used are the 
Higgins Waterproof Inks, which are made in the following colors: 
black, violet, indigo, blue, green, yellow 7 , orange, brown, brick 
red, carmine, and scarlet. In addition to being waterproof, they 
make it possible to combine curves readily without destroying 
their identity. It is also a great advantage to use the same color 
invariably for the same kind of curve. 

In plotting a curve, it is first necessary to fix the value of 
the centimeter square or interval, as well as the extreme range 
of the curve itself. For example, in the case of temperature, a 
value of 1° Centigrade is assigned to each centimeter, since the 
thermometers used read to one-fifth of a degree, which thus cor- 
responds with the value of the 2-millimeter units of each square. 
The length of the sheet permits a range of 22 degrees Centigrade, 
within which the greater number of temperature curves for a 
particular season will fall. It is very desirable that the unit 
interval and the range be the same for each factor, in order that 
all curve sheets for the same region may admit of direct com- 
parison. The major intervals are indicated at both sides of the 
sheet, and the time or the space intervals at the top. The read- 
ings upon which the curve is based are taken from the field record, 
and the proper position of each is indicated by a dot. The dots 
are first connected by a pencil line ; the curves being made angular 
rather than flowing. After being carefully checked, the line is 



36 PLANT PHYSIOLOGY AND ECOLOGY 

traced in ink. Each curve sheet is properly labeled, and 
such explanatory notes as are desirable are written upon the 
back. 

49. Intervals for the different factors. In practice, the in- 
tervals and ranges of the curves of the other factors have been 
arbitrarily fixed, as in the case of temperature. For water-con- 
tent curves each square represents a value of 2%, the smaller 
squares being 0.4%, and the range 2-48%. The unit value for 
humidity curves is taken as 5 percent, thus giving room on the 
sheet for the entire range from 1-100 percent. When a hand 
anemometer is used, curves of wind velocity are based upon the 
number of feet per minute. One hundred feet is taken as the 
unit value, and the range is from 0-2200 feet. The unit value 
for the curve of light intensity is .05. Each small square is .01, 
which permits a range of .01 to 1. on a sheet. Consequently, 
in plotting the curve of a series of habitats with a range in in- 
tensity greater than this, it is necessary to paste two sheets to- 
gether end to end. This is the usual device when the range of 
curves is too great, except when the excess is slight. In this 
case, the curve is left open at the top, and the maximum value 
is indicated at one side. All curves in combination are labeled 
at the beginning or left to indicate the level, station, or point, 
and at the end or right to show the time or day, when this is 
not the basis of the curve or series. 

Experiment 5. Determining the physical factors of habitats. The in- 
structor first chooses a series of stations comprising as many different 
habitats as can be conveniently studied. Each station, and especially 
those that fall within the same habitat, is located with respect to strik- 
ing differences of vegetation as well as physical factors. The position 
of each is 'fixed permanently by means of a stake. The number of 
stations is necessarily determined by the size of the class and the number 
of instruments available. Each observer is furnished with thermometer 
and psychrometer, and, when the variation in light warrants, with a 
photometer also. Geotomes, clinometer, compass, barometer, and 
usually also an anemometer, are carried through upon the preliminary 
survey by the class and the readings made in common. Returning 
through the series, an observer is left in each station, and the instructor 
then places himself at the proper point for signaling. Readings are 
always made in a fixed sequence. Temperature is taken first, in the 
order of air, surface, and soil; humidity follows, and finally light. To 
familiarize the student with methods, and to have a slight check upon 



THE WATER OF THE HABITAT 37 

results, it is advisable to take two or three complete sets of readings 
in rapid succession. 

A complete set of habitat readings should be taken at least once 
during the fall and winter respectively. A similar set of readings should 
be taken just before the beginning of the growing period. After the 
opening of spring, readings are taken once every week in connection 
with the field study of the development and structure of the various 
formations. Simultaneous readings through the different levels are of 
the greatest value, and are the ones regularly made. Once during the 
spring, however, it is arranged to have the class spend the whole day 
in the field for the sake of ascertaining the hourly variation of the factors. 
Likewise considerable value attaches to readings made simultaneously 
in the different points of a single station, the stations being read suc- 
cessively. Each student should enter all the readings made in his 
record-book. Representative curves and combinations should also be 
made and filed with the records. 



CHAPTER III 
ADJUSTMENT TO WATER 

50. Responses to water stimuli. The primary responses of 
the plant to the water of the habitat are four: namely, absorp- 
tion, diffusion, transport, and transpiration. Absorption is the 
response of the root to water content. Transpiration is the evap- 
oration of water from the leaf, and in some measure from the 
stem also, in response to the humidity of the air. Diffusion is 
the process by which water is carried from cell to cell through 
the various tissues. Transport is the movement of water along 
certain prescribed pathways in its journey through the stem to 
the leaves. None of these are simple processes; all involve several 
factors that will be considered in the proper place. Absorption 
is the initial activity. It is followed by diffusion and this by 
transport. The water transported to the leaves is carried to 
the various cells by diffusion and finally passes off in consequence 
of transpiration. In any living plant, all of these processes are 
ordinarily taking place at the same moment, though it is equally 
clear that transpiration must be preceded logically by transport, 
transport by diffusion, and diffusion by absorption. 

Absorption 

51. General relations. Absorption is the function by which 
water is taken into the body of the plant. It is an essential property 
of every living cell in contact with water supply, and is practically 
the same for the smallest one-celled water plant, and for the 
largest tree. Primitive water plants, i.e., the algae, such as pond- 
scums, seaweeds, etc., ordinarily use the entire surface of the 
plant body for absorbing water. Terrestrial plants, on the other 
hand, have reduced the absorbing surface as the plant emerged 

3S 



ADJUSTMENT TO WATER 39 

more and more into the air. In consequence, such plants have 
developed the part of the body in contact with the water supply 
into a special organ for absorption, i.e., the root. The funda- 
mental nature of this relation is seen clearly in those terrestrial 
forms that have come to be submerged. The absorptive organ is 
lost and the function of absorption is again distributed over the 
entire surface. 

Absorption is an inherent property of protoplasm. In terrestrial 
forms, and in flowering plants especially, it is confined to the root, 
and the form and structure of the latter have an immediate 
bearing upon this function. Absorption consists of three processes 
or factors: imbibition, osmosis, and protoplasmic attraction. 
These act in unison, but for the sake of clearness they will first 
be considered separately, following a preliminary account of the 
root. 

52. The form of roots. The most primitive terrestrial plants, 
liverworts, mosses, fern prothallia, and a few alga?, such as Botry- 
dium, possess filamentous roots or rhizoids. The higher terrestrial 
forms, the ferns and flowering plants, possess massive roots, in 
which the absorbing surface is regularly modified to form root- 
hairs. The latter are long, tube-like cells, which correspond in 
structure and function to the rhizoids of simpler plants. The 
form of the root is variously modified in response to the character 
of the habitat, and to the need of storing food material. Such 
changes are structural responses and are considered under adapta- 
tion. The structure of the root, on the other hand, is more or 
less the same for all flowering plants, and a knowledge of it is 
essential to an understanding of the functions of the root. 

53. Primary regions of the root. Roots, like stems and leaves, 
consist of three primary regions, which are distinguishable in the 
embryo, and remain more or less distinct throughout the life of 
the plant. These regions when in the condition of primary meristem 
are termed dermatogen, periblem, and plerome. The dermatogen 
or " epidermis producer" is the outermost layer of the entire 
plant body. It normally persists throughout the life of the plant 
as a single layer, owing to the fact that the division of its cells 
occurs in two planes only. The plerome is the central cylinder 
or core of the plant. It develops primarily into the fibrovascular 
system. Between the dermatogen and the plerome lies the broad 
area of the periblem, which changes largely into the cortical or 



40 



PLANT PHYSIOLOGY AND ECOLOGY 



nutritive parenchyma. In addition to these three regions, common 
to root, stem, and leaf, the root possesses a fourth, the root-cap, 
which is peculiar to it. The root-cap usually arises from the 
dermatogen of the root-tip. It consists for the most part of paren- 
chyma-like cells, which act as a cushion to protect the more delicate 




Fig. 11. — The root system of the shade and sun forms of the false Solo- 
mon's seal, Wagnera stellata. The shade form with few rootlets grows 
at the edges of brooks where the holard is 30-60%; the sun form is a 
gravel plant with a holard of 5-6%. 

meristem against tearing and crushing. As the root elongates, 
the cells of the cap are extended along the surface, where they 
gradually wear away or exfoliate. The same process occurs also 
at the lower end of the cap, causing it to wear away as it grows. 

54. Detailed structure. A section of a typical root reveals 
the three primary regions changed into epidermal, cortical, and 
vascular tissues. The epidermal layer is merely the dermatogen 
changed into a layer of permanent parenchyma-like cells. It 
consists of epidermal cells, here and there drawn out into a long 



ADJUSTMENT TO WATER 



41 



tube, the root-hair. The cortical region consists of a varying 
number of parenchyma layers. The cells, like those of the epi- 
dermal layer, have thin cellulose walls, covered on the inner surface 
by a thin layer of protoplasm. They are filled with water, but 
usually lack other inclusions, although crystals are sometimes 
present. Although the layers appear but slightly differentiated, 
the outermost and innermost differ from the intermediate ones 
in their final development. The outermost layer is the exoderm. 
Its especial task is to replace the epidermis when the latter is 
exfoliated, and accordingly to act as a root-hair producing surface. 
The innermost layer is the endoderm. It is used for storage and 
as a nutritive layer in the formation of lateral rootlets. When the 



pr> 




Fig. 12. — Longisection of a root tip of the common dock, Rumex altissimus. 
The primary regions are: d, dermatogen; pe, periblem; pi, plerome; 
ca, calyptra. The outer row of cells in the plerome is the pericycle, pr. 

other cortical layers have been exfoliated, in consequence of the 
growth of the root in thickness, the cells of the endoderm divide 
to form a several-layered corky covering for the root. 

The vascular region consists of a single nbrovascular bundle 
surrounded by a layer called the pericycle. The latter, by the 
periclinal division of several cells, produces the three primary layers 
of rootlets. By a similar division of the cells in front of the tra- 
cheids of the bundle, it completes the ring of cambium which 
makes possible the secondary growth of the root, i.e., its growth 
in thickness. The bundle of the root is of the radial type. It 
consists of woody tissue or xylem and sieve tissue or phloem. 
These are separated by a meristematic tissue termed mesenchym. 



42 



PLANT PHYSIOLOGY AND ECOLOGY 



The xylem usually occurs in narrow strands, which are united in 
the middle. The number of xylem strands is usually two or four, 
but it varies for different species. The phloem occurs in a corre- 
sponding number of plates or masses, alternating with the xylem 
strands, and lying near or next to the pericycle. The mesenchym 
gives rise to the major portion of the cambium ring necessary for 
secondary growth, and sometimes produces a pith. 

55. Origin and structure of root-hairs. The particular function 
of the epidermal layer is absorption. The cortical region has no 
special function, though diffusion is confined 
to it very largely. The vascular region 
serves as a pathway for the transport of 
water. The function of mechanical support, 
which is peculiar to the fibrovascular bun- 
dles, is entirely secondary in importance, 
owing to the support afforded by the soil. 
The epidermal layer of the roots of water 
plants shows practically no differentiation 
with respect to absorption. Practically all 
cells of the surface, except those that are 
very young or very old, absorb water with 
equal readiness. The roots of all land plants, 
except of those that grow in very wet places, 
are especially adapted to the absorption of 
water from the soil by means of root-hairs. 
The latter are not separate cells, but thread- 
like prolongations of the epidermal cells 
from which they arise. They have thin un- 
modified walls lined with a scarcely demon- 
strable layer of protoplasm. Root-hairs do 
the whole surface, but are 





Fig. 13.— Seedlings of 
white mustard, Bras- 
sica alba, grown in not arise over 

^nSeTootes ™niined to a P articular ™Z™ behind «* 

and their relation to tip which consists of meristem. As the root 
the particles of sand. elongates the i der hairs die off as new ones 
(After Sachs.) f > 

are formed, and the zone 01 root-hairs main- 
tains an almost constant width. Not all of the epidermal cells 
produce root-hairs, two or more unmodified cells standing between 
adjacent hairs. These unmodified cells are doubtless able to absorb 
water, but they can not be very active under ordinary conditions, 
since the root-hairs obtain most of the water available. Behind 



ADJUSTMENT TO WATER 43 

the zone of root-hairs the outer wall of the epidermal cells is 
rendered more or less impervious, i.e., cutinized, in order to pre- 
vent the cells from drying. A similar fate apparently overtakes 
the root-hair cells after the hair has shriveled and disappeared. 

56. Effect of water content upon root-hairs and roots. The 
formation of root-hairs is closely connected with the amount of 
water present. Water plants do not form root-hairs at all, or 
only to a small degree. The same species, indeed the same root, 
will form abundant root-hairs in moderately dry soil or in moist 
air, while in the water few or no hairs are produced. The rea- 
son for this seems evident, when it is borne in mind that root- 
hairs are primarily for the purpose of increasing the absorbing 
surface. In streams, ponds, etc., the water supply is not only 
unlimited, but the water is constantly brought into contact with 
the epidermal cells. In soils, on the contrary, the amount of 
available water is usually limited. Moreover, soil water moves 
much less readily, and consequently the epidermal cells must 
themselves move toward the water. They accomplish this by 
extending their surface in the form of a long, narrow hair. 

While the shape and position of roots are largely determined 
by water content and other soil factors, most roots possess the 
common property of growing in the direction of the greatest 
moisture. This property is termed hydrotropism, i.e., a turning 
toward water. It is not only possible to demonstrate by experi- 
ment that the growth of root-hairs is in the direction of the great- 
est water content, but it can also be shown that the root actually 
curves toward moisture. The curvature takes place in the region 
of greater growth, i.e., at a place some distance behind the tip. 
The stimulus, however, seems to be received by the tip, and is 
then transmitted to the region that is growing most rapidly. 
The advantages of hydrotropism are evident, inasmuch as it 
enables the plant practically to go in search of water at those 
times when the supply is more or less inadequate. 

The extent of the root system differs widely in different plants. 
In some species the root soon stops growing, and in consequence 
always remains poorly developed; in others the root system 
grows throughout the life of the plant or at least for a long time. 
Plants that grow in deep soil usually have more extensive root 
systems than those in shallow soils, but there are many excep- 
tions to this. Moreover, the prevalent opinion that there is a 



44 PLANT PHYSIOLOGY AND ECOLOGY 

constant ratio between the root system and the aerial part of 
the plant is not necessarily true. The actual extent of root sys- 
tems is well shown by Nobbe's results with one-year-old seed- 
lings of the pine and the fir. The former has a shallow root sys- 
tem, the latter a deep-seated one. The total length of roots in the 
pine was found to be 12 meters, and in the fir 2 meters. The sur- 
face area of the roots was 200 sq. cm. in the pine, and 40 sq. cm. 
in the fir. The estimated length of the root system of mature 
grains is 500 m., and of a large watermelon vine 25 kilometers. 
In vigorous water plants, on the contrary, the entire root system 
measures but a very few meters. 

Experiment 6. Structure of the root and formation of root hairs. 

Place seeds of the garden sunflower in two germinators, one of which 
is nearly filled with water, the other containing barely enough water 
to keep the air moist. When the roots reach 2 cm. in length, cut thin 
cross- and longisections of one or two roots from each germinator. Make 
a detailed drawing of a segment reaching to the middle of the cross- 
section, and of one extending across the longisection, indicating the 
various regions and tissues. Measure and draw a typical root-hair, 
computing also its surface. From counts of the cross- and longisections, 
estimate the number of root-hairs on a root taken from each germinator. 
With the surface of the root-hair already measured as a basis, compute 
the total absorbing area of each root. 

Experiment 7. Hydrotropism. Embed a porous tube or a 2-inch pot 
without a hole in the bottom, in the middle of a 6-inch pot filled with 
soil. Moisten the soil slightly, and plant sunflower seeds in it. Place 
the seeds equally distant from the edges of the pots in such manner that 
the emerging roots will start to grow in all directions. Fill the porous 
tube with water. A few days after the cotyledons appear, dig up the 
seedlings carefully, noting the position of the roots, and the region in which 
the turning occurs. 

57. Imbibition. This is the process that takes place when a 
solid or semi-solid absorbs water. It is due to the attraction 
which the molecules of the two substances have for each other. 
This in some degree overcomes the cohesion of the molecules 
of the solid, causing them to separate and the solid to swell. The 
swelling due to imbibition can be readily measured, and during 
the process it can be easily shown that a large amount of work 
is being done. The swelling due to imbibition is most pronounced 
in the dead or resting tissues of plants, but it can also be detected 
in living cells. 



ADJUSTMENT TO WATER 45 

Imbibition is a factor in absorption, since it affects both the 
cellulose wall of the root-hair, and the lining membrane of pro- 
toplasm. It not only carries the water into the plant by filling 
wall and protoplasm with it, but it also renders both wall and 
protoplasm more porous by reason of the swelling due to the 
separation of the molecules. For this reason it is a most important 
aid to the process of osmosis, which controls the diffusion of liquids 
through plant membranes. Imbibition takes place more rapidly 
at high than at low temperatures. It is increased by the action 
of acids and alkalies, provided they are not too strong. It is 
greater in distilled water than in water containing salts in solu- 
tion. The powerful force exerted in imbibition is made evident 
by the lifting power of swelling seeds or wood, as well as by the 
fact that heat is given off. This evolution of heat is apparently 
due to the movement of the molecules and also to the conden- 
sation of the water imbibed. The turning and twisting of awns 
when moistened, as in Stipa, is likewise an evidence of the energy 
of imbibition. 

Experiment 8. Water of imbibition. Put 25 grams of dry peas and 
an equal amount of distilled water in a small cylindrical jar with straight 
walls. Cover the peas with a close-fitting cork float, through the center 
of which is placed a thermometer. Weight the cork down with several 
100-gram weights, at the same time marking its position. Note the 
rise of the float as the peas swell, and also compare the temperature 
with that of the surrounding air. Determine the volume of a swollen 
and a dry pea by means of calipers, and compute the percent of increase 
due to imbibition. 

Cut small pieces of hard wood and of soft wood to the same size. 
Measure carefully, soak in distilled water, measure again, and compare 
the amount of swelling in the two. 

58. Osmosis. Two solutions of unequal density when sepa- 
rated by a porous membrane will gradually pass through the lat- 
ter, and mix with each other. This purely physical process of 
diffusion is termed osmosis. It depends primarily upon the dif- 
fering density of solutions able to wet the limiting membrane. 
Osmosis consists essentially of two currents which set in oppo- 
site directions. The first current set up is toward the denser 
or stronger solution, but this is regularly compensated by a second 
current passing from the denser to the weaker solution. The 
difference in density between the two solutions determines the 
strength of the first current, as well as the length of interval that 



46 PLANT PHYSIOLOGY AND ECOLOGY 

elapses before the second is set up. This is largely determined 
also by the affinity for water possessed by the substances in solu- 
tion. For example, sugar has a comparatively slight affinity for 
water, and is much less active in osmosis than mineral salts or 
organic acids, which show a greater attraction for water. 

59. Osmosis in root-hairs. The conditions presented by a 
root-hair embedded in a moist soil are apparently those found 
in ordinary osmosis. It offers a porous cellulose wall wetted 
within by a cell sap denser than the soil water which wets it 
throughout. A moment's reflection, however, makes it clear 
that in such event the root-hair would sooner or later lose as 
much water as it absorbs. The plant would then be unable to 
get water for its various functions, especially transpiration, and 
it would quickly wilt and die. Evidently, the root-hair must 
be enabled to modify ordinary osmosis in such a manner that it 
may take in more water than it gives out. This absolutely indis- 
pensable modification of osmosis is due to the presence of the 
protoplasmic membrane which lines the hair. This membrane, 
like all protoplasm, has a great attraction for water. Its effect 
is to increase the strength of the first current in osmosis (endos- 
mose) and to decrease the return current (exosmose). The latter 
indeed becomes practically imperceptible. It is represented by 
the slow passage of a minute quantity of acid cell sap, indicated 
by the reaction of roots to litmus paper, or by their etching effect 
upon a marble surface. Osmosis in the root-hair differs from 
ordinary physical osmosis in that it consists practically of the 
inward current alone, the return current being prevented by the 
active properties of the protoplasm. Within the root, the nor- 
mal process with currents in both directions takes place between 
the cells of parenchyma. 

The osmotic properties of root-hairs are due to the fact that 
the density of the cell-sap is normally greater than that of the 
soil water. The substances in the hair capable of inducing os- 
mosis are the sugars used as food, the mineral salts absorbed 
from the soil, and the organic acids and their salts. Of these, . 
the sugars have little or no influence; the mineral salts play a 
relatively small part, since they can be obtained only by absorp- 
tion from without. The organic acids and salts constantly pro- 
duced by the activities of the cell are by far the most active, 
and it is to them that osmosis is chiefly due. 



ADJUSTMENT TO WATER 47 

60. Influence of soluble salts. The composition and density 
of the cell sap naturally varies in different plants. In all ordi- 
nary habitats the density is greater than that of the soil water, 
since this is one of the necessary conditions of end osmose. The 
density of the water content is determined by the amount of soluble 
salts present in the soil, and by the amount of water applied to it. 
The water content of ordinary soils contains from 0.01% to 0.1% 
of dissolved salts. In alkaline lands the soluble salts vary from 
0.2% to more than 3%. Up to 0.2%, the action of such salts 
is not injurious, as indicated by the growth of ordinary field crops 
upon them. Beyond this point concentration becomes more 
and more injurious, owing to the difficulty of absorption. The 
limit for the most resistant cultivated plants is reached at about 
1%, and beyond this only such alkaline plants as the salt bush 
and the greasewood are able to grow. Soils containing much 
sodium carbonate and bicarbonate (black alkali) render osmosis 
and absorption much more difficult. The injurious effects of 
such salts become evident at a concentration of 0.05%, while only 
the more resistant plants can withstand 0.1% to 0.2%. 

Experiment 9. Demonstration of osmosis. Tie a piece of parchment 
or dialyzer paper over the bulb of a thistle tube, taking pains to make 
it fit tightly. Fill the bulb with a 25% or 30% solution of common 
salt and allow the latter to rise a short distance in the tube. Place 
the bulb in a beaker filled with distilled water and support it by means 
of a ring-stand. Follow the rise or fall of the column in the tube, and 
mark the various heights. Test the distilled water from time to time 
by means of a drop of silver chloride to determine whether the salt 
has passed into it. When the column has reached the highest 
point, place the bulb in a concentrated salt solution, and note the 
behavior. 

Experiment 10. The effect of soluble salts. Germinate sunflower 
seeds in three pots filled with sawdust, and after the seedlings are well 
established, water one with distilled water, the second with a nutrient 
solution, and the third with a 5% solution of common salt. Compare 
the behavior of the seedlings, and note also the effect of the solutions 
upon the root-hairs. 

Treat a thread of pond scum with 1% solution of common salt 
colored with methyl blue. Note the effect, and after a few minutes 
replace the salt solution with distilled water. This shrinkage of the 
protoplasm, which is known as plasmolysis, is essentially what occurs 
in the root-hairs in soil watered with the salt solution. 



48 PLANT PHYSIOLOGY AND ECOLOGY 

61. Effect of protoplasm upon the absorption of soluble salts. 

The protoplasmic membrane of the cell is not merely a powerful 
factor in determining the absorption of water. It also exerts 
an extremely important action upon the amount of soluble mate- 
rial taken in with the water. The latter is not absorbed as it 
is found in the soil. The protoplasm has a different degree of 
attraction for the molecules of water and for those of the dis- 
solved salts. Moreover, it has a greater affinity for some salts 
than for others. In consequence, the soil water that enters the 
plant not only contains a changed amount of soluble salts, but 
the relative amounts of these are also different from the amounts 
present in the soils. The rule apparently is that proportionately 
more water than salt is absorbed when the solution is relatively 
concentrated, and proportionately more salt than water when 
the solution is dilute. An analysis of the ash of species growing 
in the same habitat not only shows that soluble salts exist in 
the plant in a different proportion from that found in the water, 
but also that this proportion is different in each species. This 
power of the protoplasm to take up water and soluble salts with- 
out strict regard to their proportion is of very great importance 
to the plant. The plant is enabled to filter out as it were some 
of the dissolved salt when the concentration of the latter is higher, 
and it is unnecessary or injurious. On the other hand, the root- 
hairs are able to absorb proportionately more nutrient material 
when the solution is dilute, thus avoiding the necessity of absorb- 
ing an excess of water in order to secure the necessary amount 
of salts. Furthermore, this property enables the protoplasm to 
absorb more of the salts that are being used most, and less of 
the others. Indeed, the property is probably to be ascribed to 
the difference in the demands of the plant. 

62. Diffusion. The passage of absorbed soil water from the 
root-hair cell to a neighboring cell, and from this to the other 
cells of the root, takes place by virtue of osmosis. The absorbed 
water first passes into the cortical cell, and in return a certain 
amount of cell sap passes into the cell of the root-hair. This 
process of diffusion is continued throughout the cortical region 
until the water taken in by the root-hairs is finally brought to 
the fibrovascular region. Diffusion, however, is not due to os- 
mosis alone. The attraction of protoplasm for water, as well as 
for other substances that it needs, doubtless plays a part. In 



ADJUSTMENT TO WATER 49 

addition, the entrance of soil water into the root-hair cell causes 
the elastic wall of the latter to stretch. The tendency of the 
stretched wall to recoil reacts upon the enclosed sap. Since the 
protoplasmic membrane prevents the escape of the water out- 
ward, the latter is force I through into the adjoining cells, where 
the pull of one membrane is balanced by that of another. 

There is no definite pathway for diffusion between the absorb- 
ing surface and the fibrovascular bundle. Diffusion may take 
place in all directions through a tissue. The general direction is 
determined, however, by the location of the region in which there 
is a lack of the diffusible substances. In the root the need for 
the water of absorption increases toward the fibrovascular bundles 
which are engaged in transporting it upward to the leaves. In 
similar fashion, the demand for the food materials brought down 
by the sieve tissue increases toward the outer layers. The gen- 
eral direction is determined by these two facts, and in the root 
diffusion is predominantly in a radial direction. It is not cer- 
tain that diffusing substances follow the shortest route between 
surface and bundle, but this seems probable. 

Experiment n. Diffusion in liquids and in tissues. Fill a 10-cc. 
cylindrical graduate half full of a 5% solution of common salt colored 
with methyl blue, and with a pipette carefully place 5 cc. of distilled 
water colored with tropaeolin above the salt solution. Note the rate 
of diffusion. 

Cut from a turnip a strip of tissue a decimeter long and a centimeter 
wide and deep. Place the strip in a 1% solution of common salt colored 
with methyl blue. Note the rate of diffusion by cutting off segments 
of 2 cm. from time to time. 

63. Turgidity. When water is absorbed by the root-hair, 
the increased pressure within the cell forces the protoplast still 
more firmly against the wall, and at the same time stretches the 
latter. The elasticity of the wall leads to a recoil against this 
force, and as a result the whole cell becomes firm and rigid. 
The same phenomenon, which is termed turgidity, takes place 
in all of the cells of the cortical parenchyma. It is necessarily 
absent in cells whose walls have lost their elasticity, such as the 
fibres and vessels of the bundles, and also in cells without pro- 
toplasm. The turgidity of each parenchyma cell renders the 
whole cortical region turgid. The turgidity which is thus given 
to the entire root is further emphasized by the tendency of the 



50 PLANT PHYSIOLOGY AND ECOLOGY 

more rigid bundle and epidermal layer to compress the swollen 
parenchyma and hence to increase its rigidity. In consequence, 
turgidity becomes a fundamental factor in support. This is 
particularly evident when stems and leaves are allowed to wilt. 
The water is removed from the cells more rapidly than it is sup- 
plied. The cells collapse, and with them the stem and leaves, 
in spite of the support of the fibrovascular bundles. 

Turgidity is so intimately wrapped up with the absorption 
of water and its diffusion by means of osmosis that plants can 
function normally only while they are turgid. A temporary loss 
of turgidity is usually not fatal, but all the functions dependent 
upon water are necessarily brought to an abrupt stop. If the 
flaccid condition continues for a long period, the power of the 
plant to carry on absorption and diffusion is lost, and the plant 
dies. In this connection, turgidity renders the indispensable ser- 
vice of keeping the protoplasm firmly pressed against the cellu- 
lose walls, a condition necessary for keeping the wall filled with 
water. The latter in turn is absolutely necessary to the passage 
of water through the wall, and hence to diffusion. Turgidity 
is thus seen to furnish a ready clue to the condition of the plant. 
Turgid plants are normal, flaccid plants abnormal, i.e., in a patho- 
logical condition. 

Experiment 12. Demonstration of turgidity. Cut a 3-inch section 
of dialyzer tubing and soak it in distilled water until softened. Carefully 
fold and tie one end, fill with a 10% solution of common salt, and tie 
the open end as closely as possible above the level of the liquid. Place 
this artificial cell first in distilled water, and, after it has become fully 
distended, put it in a 20% solution of common salt. Note the results. 
Place the cell in soil saturated with distilled water colored by erythrosin, 
and explain its behavior. 

Allow two sunflower plants to wilt. Water one as soon as the 
leaves wilt strongly, and the upper part of the plant begins to droop. 
Water the second only after the whole plant has collapsed. Explain 
the results. 

TRANSPORT 

64. General nature. In all stemmed plants, the surface that 
absorbs water is separated by a greater or less length of stem 
from the leaf surface which loses water. Even in stemless flowering 
plants, practically the same condition exists, since a similar separa- 



ADJUSTMENT TO WATER 51 

tion of absorbing and transpiring surface's is effected by the petiole. 
By far the greater bulk of the water absorbed must be carried 
through the stem to the leaves. The rapid movement of the large 
amount of water which is lost by the active evaporation from the 
leaves makes the presence of a special pathway imperative. Such 
a pathway is found in the fibrovascular system, which has already 
been seen in the root to occupy a position peculiarly advantageous 
with respect to the diffusing water. The fibrovascular bundle 
of the root connects directly with those of the stem, and the bundles 
of the stem are prolonged in the form of minute veins to all parts 
of the leaves. The fibrovascular system thus resembles a con- 
tinuous series of water pipes, serving to collect the water absorbed 
by the roots, to carry it with relatively slight loss through the 
stem, and to distribute it to all parts of the leaf. This movement 
of the water, which is an essential function of stems, is transport. 
The details of transport will be more readily understood after a 
consideration of the structure of stems. 

65. Types of stem structure. Among flowering plants, dicoty- 
ledons possess a stem essentially different in structure from that 
of monocotyledons. Though in less degree, woody stems also 
differ characteristically from herbaceous ones. All of these types 
are in fundamental agreement, inasmuch as each possesses the 
three primary regions corresponding to dermatogen, periblem, and 
plerome, though in monocotyledons the regions are confused. 
The essential differences between the three kinds of stem hinge 
upon the nature and arrangement of the fibrovascular bundles. 
The bundles of a dicotyledonous stem possess a layer of meristem, 
or cambium, and are able to increase their size. They are usually 
arranged in a more or less circular row placed about midway 
between the center and the surface of the stem. The bundles of 
monocotyledons have no cambium, and are consequently unable 
to grow after they are once formed. They are scattered more or 
less uniformly throughout the stem. The bundles of fern stems 
are also without cambium, and are solitary or scattered. Woody 
stems are primarily dicotyledonous or monocotyledonous. They 
differ from herbaceous stems of the same type in the excessive 
development of woody tissues. This is shown in our common 
shrubs and trees, all of which are dicotyledons. In these the 
number of bundles is repeatedly augmented by the formation of 
new bundles from the ring of cambium, until the fibrovascular 



52 PLANT PHYSIOLOGY AND ECOLOGY 

system is a closed ring, encircling the pith. The continued growth 
of this ring from year to year results in a stem consisting almost 
wholly of woody tissues. 

66. Stem structure of an herbaceous dicotyledon. A section 
of the stem of an herbaceous dicotyledon reveals an epidermal, 
cortical, and fibrovascular region. The former is regularly a single 
layer of cells, as in the root. Its cells are often differentiated into 
epidermal cells, hairs, and guard-cells of stomata. The epidermal 
cell has its outer wall rendered impervious, and often also thickened, 
to prevent water loss. Hair-cells are epidermal cells elongated 
after the manner of root-hairs. They are usually many-celled, 
however, and their walls are cutinized, indicating that they are a 
protection against water loss. Guard-cells are indirectly a result 
of the cutinization of the epidermis. The latter renders necessary 
the presence of openings for admitting gases, and the consequent 
danger of water loss requires the development of guard-cells to 
regulate the opening. All of these modifications of the epidermis 
are really characteristic of the leaf, and they occur in the stem 
only in so far as it assumes some of the same functions. 

The cortical region often consists of parenchyma alone. In 
many plants, however, a secondary supportive tissue, the collen- 
chyma, is developed next the epidermis, and, more rarely, one or 
more rows of bundles of wood or stone fibers appear somewhere 
in this region also. Milk-tubes are likewise found here. The 
parenchyma is usually characterized by the presence of chloro- 
plasts, though usually to a less degree than that of the leaf, and 
of intercellular spaces for the passage of air. In most cases, it 
does not show a differentiation of exoderm and endoderm, though 
the latter often occurs as an incomplete sheath about the fibro- 
vascular bundles. The fibrovascular system is usually in the 
form of an interrupted circle of bundles strung like beads upon 
the ring of cambium. In older stems, or in robust species, such 
as the sunflower, the ring of bundles often becomes completely 
closed. In a few cases, such as the cucumber, there is a secondary 
outer ring. In many succulents, and in water plants, the fibro- 
vascular bundles are greatly reduced, and their behavior excep- 
tional. As in the root, each bundle has a phloem and a xylem 
portion, though their relative position is now changed. They are 
no longer radial, but the phloem has swung around so that it is 
opposite and outside of the xylem. In most herbs, the phloem 



ADJUSTMENT TO WATER 53 

or sieve area of the bundle consists of sieve tubes, companion cells, 
and thin-walled fibers. The sunflower and similar woody herbs 
possess in addition bundles of thick-walled fibers, or bast fibers, 
located on the outside of the sieve tissue. 

Between the phloem and the xylem lie several or many layers 
of clear, thin-walled seriate cells, usually referred to as cambium. 
Properly speaking, the true cambium is but a single layer of active 
meristem, each cell of which by division alternately forms a xylem 
and a phloem cell. Since the latter are modified but slowly into 
the various parts of the bundle, there are always a number of 
layers which closely resemble the mother cells. The xylem com- 
prises wood fibers and tracheary vessels, the latter of various 
types, ringed, spiral, reticulated, and pitted. Occasionally, also, 
large, pitted wood fibers occur here and there: these are called 
tracheids. The tracheary vessels are usually found in the middle 
of the xylem, surrounded and separated by the wood fibers. In 
the longisection it should be noticed that the vessels are continuous 
tubes of great length, while the wood fibers are single closed cells. 
Within the fibrovascular ring, and derived like it from the plerome, 
lies the pith, consisting usually of inactive parenchyma, some- 
times used for storage. 

67. Stems of monocotyledons. A section of the stem of an 
herbaceous monocotyledon shows essentially the tissues found in 
the dicotyledon. The cambium, however, is found only near the 
periphery, and the bast is replaced by a sheath of stone fibers 
which surrounds each bundle. The irregular arrangement of the 
bundles likewise makes it impossible to distinguish a cortical region 
or the pith. 

68. Structure of woody stems. A woody dicotyledon has 
essentially the grouping of tissues found in the herbaceous type. 
Wood and bast have been greatly emphasized at the expense of 
other tissues, and a new tissue, the cork, has been added. A 
section of a typical woody stem shows on the outside the cork, 
together with certain cambium cells which produce it. Beneath 
the cork are a few layers of collenchyma, followed by a broader 
area of cortical parenchyma. Within the latter follow the bast 
bundles, parenchymatous cells, sieve tubes, and companion cells 
of the phloem. Beyond the latter is the cambium, and the rings 
of wood surrounding the pith. The wood consists chiefly of 
tracheids and wood fibers, together with a larger or smaller number 



54 PLANT PHYSIOLOGY AND ECOLOGY 

of vessels, and short wood-parenchyma cells, usually rilled with 
starch. Arising from the cambium and running radially through 
the bast and wood are the medullary rays. These consist of rows 
of parenchyma cells elongated radially. They serve as pathways 
for food solutions passing to and from the storage-cells of the pith, 
and as storage-cells for starch. 

69. Functions of the stem. The stem is primarily an organ 
for support. Its most important secondary function is the trans- 
port of the water taken up by the root. The structural features 
peculiar to the stem are to be found in the arrangement and com- 
position of the fibrovascular bundles. In the monocotyledons 
the need for increased support is met by the formation of new 
bundles. Dicotyledons can attain the same end by increasing 
either the number of bundles or the size of each bundle. Woody 
herbs, shrubs, and trees unite both methods, first increasing the 
number of bundles as well as the size, and later, after the ring 
becomes closed, increasing the size alone. In herbaceous plants 
practically all of the elements of the xylem play a part in the 
transport of water, but in woody plants the thick-walled fibers 
have little or no part in this function. 

Experiment 13. Structure of stems. Cut thin cross- and longi- 
sections of sunflower, corn, and ash, the first two preferably by means 
of the paraffin method. Make a schematic drawing of the cross-section 
showing the fibrovascular system. Draw a segment from the cross- and 
longisection of each stem under the high power, and indicate the various 
regions and elements. 

70. The upward movement of the water. When the water has 
been brought by diffusion to the radial bundle of the root, osmosis 
ceases. This is due to the fact that the elements of the bundle, 
fibers, tracheids, and vessels, are dead cells which lack protoplasm 
and for the most part have rigid and thickened Avails. Under 
these conditions, osmosis and consequent diffusion become im- 
possible, and imbibition and infiltration take their place. These 
in themselves are slow processes, and require to be emphasized 
in order that? water may enter the bundle more rapidly. This 
emphasis is furnished by the cumulative action of the turgid cells 
of the cortical parenchyma. The constant absorption of water 
by the root-hairs, and its diffusion through the parenchyma, put 
an increasing strain upon the elastic wall of each cell and upon 



ADJUSTMENT TO WATER 55 

the epidermal layers. This strain greatly increases the imbibition 
and infiltration from the parenchyma cells adjoining the bundles, 
and explains how the water may enter rapidly enough to supply 
the constant evaporation from the leaves. 

Infiltration occurs more easily and rapidly through thin walls. 
Consequently water passes readily into the vessels and tracheids 
through the thin areas and the pits of the wall. It enters the 
thifkened walls of wood and stone fibers only with much difficulty 
or not at all. In addition, tracheids are separated by thin end 
walls, and vessels are continuous for long distances, w r hile fibers 
are connected by thickened walls. From this it becomes evident 
that the movement of water upward must take place very largely 
in the vessels and tracheids, and to a very slight degree, if at all, 
in the thick-walled fibers. It will be seen later that the sieve 
tubes of the bundle have a special function, and hence are not 
available for carrying water to the leaves. 

71. Causes of the movement. The force that arises from the 
turgidity of the cortical parenchyma is termed root pressure. 
The latter is often if not regularly sufficient, not only to force 
the water to filter into the tracheids and vessels, but also to cause 
it to rise some distance in them. Under exceptionally favorable 
circumstances, and with vigorous plants, this effect of root pressure 
may be demonstrated. A stem is cut off near the ground, and a 
glass tube of small bore is fitted over it tightly by means of a rubber 
joint. If the plant is absorbing water strongly, the root pressure 
causes the latter to rise in the tube. Once within the tracheids 
and vessels, the water is subject to the effect of capillary action. 
Capillarity, which is merely one form of attraction, is the force 
which causes a column of water to rise in an extremely fine glass 
tube when the lower end is placed in water. Capillary action is 
due to surface tension, i.e., to the attraction between the molecules 
of the tube and those of the water. It is exerted in the cavity 
of the vessels, but attraction also causes water to rise in some 
degree in the walls of both tracheids and vessels. A lifting effect 
upon the water in the bundles is doubtless exerted by the trans- 
piration of the leaves. This effect is largely due to the active force 
of osmosis arising from the increased osmotic pressure in the leaf- 
cells. The latter is brought about by the increased density of 
the cell-sap caused by evaporation. A similar force is probably 
exerted by the osmotic pressure of the cells of the stem which are 



56 PLANT PHYSIOLOGY AND ECOLOGY 

contiguous to the bundle. In part also this effect comes from the 
evaporation of water from cavity and wall, which facilitates the 
rise of water in both tracheid and vessel. 

The principal cause of the upward movement of water is un- 
known. The most divergent views are held, not one of which has 
proved capable of satisfactory demonstration. In the account 
just given of probable factors in the movement, it is hardly possible 
to deny the existence of root pressure, capillarity, and the lifting 
power of evaporation and osmotic pressure. The relative impor- 
tance of these, the manner in which theywork, and the existence 
of other factors are points that it is impossible to settle at present- 
It seems certain, however, that the so-called " vital properties" 
of the plant, apart from the physical forces already mentioned, 
have no part in the movement. The effect of transpiration upon 
the rate of movement would seem to bespeak great importance for 
this factor, though it is hard to prove this. In fact, while it is 
difficult to discover forces of strength sufficient to carry water 
to the tops of the tallest trees, the forces already discussed seem 
entirely adequate in the case of all herbaceous plants. 

Experiment 14. Pathway and rate of movement. Cut three sunflower 
plants in such a way as to obtain three leafy stems \, 1, and 2 dcm. long 
respectively. Cut similar leafy shoots from the young growth of a 
tree or shrub. For use as a check, cut a 2 dcm. stalk from sunflower 
and tree, and remove the leaves. Fill a flat dish with a 1% solution 
of erythrosin, and place the lower ends of the cut stems in the stain. 
Arrange the sunflower stalks in one row, and the shoots in a second 
one to permit ready comparison. Note the time that elapses before 
the stain appears in the petioles of the uppermost leaves. Compare the 
rates in herbaceous and woody stems, and in the leafy and leafless ones. 
Cut cross-sections near the base and tip of stalk and shoot, and also 
across the leaf. Draw a stem bundle, shading the elements in which 
the stain moved. Make a diagrammatic drawing of stem and leaf 
section, showing the number of pathways. 

TRANSPIRATION 

72. The structure of a representative leaf. The usual type 
of leaf is flat, broad, and thin, and is ordinarily more or less hori- 
zontal in position. The leaf is an organ peculiarly modified for 
the reception of light and the absorption of gases. Consequently, 
it becomes also the main seat of water loss or transpiration. The 



ADJUSTMENT TO WATER 



57 



cross-section of a leaf shows the three primary regions, epidermal, 
cortical, and vascular. Each of these is modified in a manner 
characteristic of the leaf. In the epidermis this modification is 
the stoma, in the cortical region the differentiation of the green 
tissue or chlorenchym, and in the vascular region the great reduc- 
tion and division of the bundles. The upper and lower epidermis 
of a leaf are very similar. In the vast majority of leaves, they 
consist of a single layer of cells. Because of a difference of expo- 
sure, the tendency of the upper epidermis is to develop cutinized 




Fig. 14. — Cross-section of the leaf of the monks-hood, Aconitum colum- 
bianum, showing the palisade tissue above and the sponge tissue 
below. 



hairs, and that of the lower to develop stomata. Thus while hairs 
and stomata often occur on both surfaces of a leaf, hairs are 
often more numerous upon the upper surface than upon the lower, 
while stomata are regularly more numerous upon the lower. In 
many leaves, hairs are found only upon the upper epidermis, 
while in others, stomata occur only upon the lower. The epi- 
dermal cells proper have their outer walls cutinized and usually 
thickened also. In these likewise the outer wall or cuticle is 
generally thicker on the upper surface than upon the lower. 

73. The chlorenchym. As a rule, the cortical region of the 
leaf consists wholly of parenchyma cells filled with chloroplasts. 
From its nature, this tissue is called chlorenchym. The latter 
comprises two distinct parts, viz., the palisade parenchyma and 
the sponge parenchyma. In the normal leaf, the palisade tissue 
occurs in the upper half, and the sponge tissue in the lower half. 
The probable causes of this differentiation are discussed under 
Light. At present it is sufficient to point out that the position 
and development of these tw r o tissues are directly connected with 
differences in the degree of exposure to light and humidity 



58 PLANT PHYSIOLOGY AND ECOLOGY 

shown by the two surfaces. The palisade tissue consists of rec- 
tangular cells elongated at right angles to the surface, and packed 
so closely in rows that the intercellular air-passages are scarcely 
visible. The sponge cells, though usually irregular in outline, 
are more or less elongated in the direction of the surface. They 
are loosely connected, and their irregular forms permit the pres- 
ence of numerous large air-spaces. The relative amounts of 
palisade and sponge tissue in the leaf are determined by water 
and light, and a further discussion of this matter will be found 
under Adaptation. 

74. The reduced bundles. The repeated division of the fibro- 
vascular bundles that enter the leaf is adapted to meet the in- 
creased need for support caused by its form. At the same time 
it serves to carry water to all parts of the leaf, which is the organ 
that needs water most. A close examination of a reduced bundle 
in section shows its intimate relation to the cells of the chlorenchym. 
The supportive elements of the bundle are greatly reduced. In 
many instances spiral vessels alone remain, thereby greatly facili- 
tating the passage of water from the bundles into the cells of the 
chlorenchym. 

Experiment 15. Structure of a leaf. Cut thin cross-sections of a 
sunflower leaf, preferably by the paraffin method. Under the high power 
draw in full detail a segment across the leaf at a point where a small 
bundle occurs. Pay especial attention to the air-spaces and the number 
and position of chloroplasts in both palisade and sponge tissue. 

Strip a bit of epidermis from each surface. Count the stomata in 
two or three fields, and compute the number for a square centimeter of 
each surface. Estimate the surface of a leaf by first weighing the 
entire blade, and then two or three pieces of it a centimeter square 
taken from different portions, and dividing the first weight by the 
average weight of a piece. The quotient is the surface in square centi- 
meters. Estimate the number of stomata on each surface of the entire 
leaf. 

75. Diffusion in the leaf. The water absorbed by the roots is 
carried throughout the leaf by the reduced bundles. The water 
passes from the vessels into the cells of the chlorenchym by rea- 
son of the osmotic pressure of the latter reinforced by the attrac- 
tion of the protoplasmic membrane. The latter effect is due to 
the fact that the vessel has no protoplasm to counteract this pull. 
The water passes from cell to cell by diffusion, exactly as in the 



ADJUSTMENT TO WATEB 59 

root. Diffusion in the leaf is due to osmotic pressure, arising in 
part from the active production of organic acids and salts in the 
cells, but chiefly, it would appear, from the increased density 
of the sap caused by evaporation. The latter reason doubtless 
causes diffusion to set most strongly toward those areas in which 
evaporation is greatest. Here, as elsewhere in the plant, diffusion 
currents are always in the direction of greatest use. 

76. Transpiring surface. In ordinary leaves, especially those 
found in the sunshine, the cutinized wall of the epidermal cells 
either entirely prevents transpiration from them or reduces it 
to an insignificant amount. The transpiring surface, therefore, 
is not the epidermis of the leaf, but it is formed by the cells that 
lose water rapidly and in relatively large amounts. It is composed 
of the aggregate cell surfaces that border on air-spaces, both in 
the sponge and the palisade tissue. At these places, the cell-sap, 
which fills the cell walls, passes into vapor whenever the air in 
the passages is not completely saturated. The moist air that 
fills the spaces gives up some of its moisture through the stomata 
to the drier air outside. It seems probable, however, that a 
more important factor in water loss is the passing of the moist 
air itself through the stomata, owing to the constant movement 
of leaves in the wind. In this case, drier air at once passes in to 
take its place. Consequently, while the number, size, and position 
of the stomata determine the ease and rapidity with which air 
and moisture pass out, the stomata do not form part of the trans- 
piring surface. On the outside, the guard-cells are protected 
against evaporation in exactly the manner of epidermal cells, 
and the surfaces next the opening are also cutinized. The inner 
surface of the guard-cell next the air-chamber is usually exposed 
to the air of the latter, and consequently contributes very slightly 
to the transpiring surface. 

Experiment 16. Measurement of the actual transpiring surface. Find 
the linear extent of the air-spaces shown in the drawing of a sunflower 
leaf by attaching a needle-point to a thread, and allowing the thread 
to run out as each side of an air-space is measured. If the result thus 
obtained is squared, it will represent roughly the area of transpiring sur- 
face for a square segment of the leaf of the width of the area studied. 
The two surfaces of this segment would represent the corresponding 
leaf surface. Determine the ratio between the actual leaf surface and 
the transpiring surface. 



60 



PLANT PHYSIOLOGY AND ECOLOGY 



77. Structure and position of stomata. The simplest form of 
stoma is a hole, which remains constantly open, and is surrounded 
by peculiar epidermal cells. In all vascular plants, however, the 
opening is regulated by two guard-cells. Below the opening regu- 
larly occurs an air-space of variable size, the air-chamber. The 
guard-cells vary considerably in different plants, but in all they 
agree in being oblong or cylindrical cells, bent in such fashion that 
they join broadly at the ends, but are free at the middle. The 
outer wall of each guard-cell is thickened, and in many cases the 
inner wall also. The wall next the opening is likewise more or 
less thickened, while that opposite the opening, i.e., touching the 
epidermal cell, is the thinnest. Guard-cells are almost invariably 
filled with chloroplasts which contain starch. 

Stomata usually occur singly, scattered more or less uniformly 
over the epidermis, but in some plants they are found in groups. 




Fig. 15. — Distribution of stomata in the epidermis of an orchid, Calypso 
borealis. The lower epidermis (1) has 36 stomata per square milli- 
meter of surface, the upper (2) but 2 per square millimeter. 

Though commonly on a level with the epidermal cells, stomata 
are often sunken for protection, either singly, as in the century 
plant, or in groups, as in the oleander. Sunken stomata usually 
possess an outer chamber or court, formed by the over-arching 
of the epidermal cells. Their guard-cells usually exhibit one or 



ADJUSTMENT TO WATER 61 

two pairs of valves, though these are often found in other stomata 
also. As already indicated, stomata are ordinarily more abundant 
upon the lower epidermis of horizontal leaves, and in some species 
are restricted to this surface. In many aquatic and rosette plants, 
the stomata are more abundant upon the upper surface, and in 
floating plants they occur on this surface alone. The leaves of sub- 
merged plants normally lack stomata. In forms more recently 
submerged, the latter sometimes persist, but are functionless. 

78. The functions of stomata. In their simplest form, stomata 
are for the purpose of permitting the ingress and egress of carbon 
dioxide and oxygen, though moisture must also pass out through 
them. In the thalloid liverworts, growing closely pressed upon 
moist earth, the danger of drying out through the openings alone 
would seem small. In leafy stemmed plants, this danger is greatly 
increased, and has necessitated the development of guard-cells. 
The latter consequently have charge of the secondary function of 
stomata, which is to regulate the amount of transpiration. The 
movements of the guard-cells are regulated by light and by the 
interaction of humidity and water content. Stomata open in 
strong light and close in weak light; consequently they show a 
periodic movement, opening in the morning and closing at night. 
When transpiration tends to exceed absorption, as in the case of 
a great decrease in humidity, the guard-cells close. This checks 
transpiration, and usually enables the roots to meet the deficiency. 
When this occurs, or when the water supply is renewed, as by a 
rain, the guard-cells open. 

79. Movements of guard-cells. The movements of the guard- 
cells are brought about by changes in their turgidity. Stomata 
close when the plant becomes flaccid, i.e., when the plant is losing 
more water than it absorbs. They open again when the plant 
becomes turgid in response to increased absorption or decreased 
transpiration. Generally speaking, stomata are open for the 
exchange of gases, when the danger from excessive water loss is 
slight, and they remain closed when the danger is great. As a 
matter of fact, the closure is rarely quite perfect, so that some 
moisture escapes even when the stoma is closed. 

The mechanism by which the stomata open and close is a simple 
one. It is most readily understood by comparing the top of a 
stoma with a cross-section of one. The thinnest wall of each 
guard-cell is the one next the epidermal cell, the others, particularly 




62 PLANT PHYSIOLOGY AND ECOLOGY 

the upper and outer, being more or less thickened. Since the guard- 
cell is active, it has a relatively high osmotic pressure, and draws 

water readily from the adjoin- 
ing epidermal cells. The result 
is to cause the inner walls of each 
guard-cell to become more and 
more convex. Since the guard- 
cells are firmly joined to each 
other at the ends, the increased 
Fig. 16. — Diagram of the stoma of turgidity forces them apart in 
Helleborus The shape of the guard- the centerj as though each were 
cells when the stoma is open is shown ' ° 

by the heavy lines. The thick outer pulled by a string attached to the 
and inner walls are shown at a and iddl " f th inner ^ Ag 
cd. (After Schwendener.) 

long as the plant remains fully 

turgid, the stomata stay open, except of course for the regular 
closing at night. When the water loss tends to become excessive, 
or the water supply deficient, the osmotic pressure of the epidermal 
cells exceeds that of the guard-cells. Water is withdrawn from 
the latter, the inner wall becomes less convex, and releases the 
strain upon the two guard-cells, which close in consequence. 
Closure in its turn is maintained until the usual turgidity is re- 
stored. In many cases it is probable also that the epidermal cells 
adjoining the stomata aid in this process by their shape and move- 
ment. 

Experiment 17. Movement of guard-cells. Strip a small piece of 
epidermis from the leaf of a turgid and of a wilted sunflower. Immerse 
the strips for a few minutes in a killing solution containing osmic acid 
(preferably Flemming's solution), wash and examine the stomata under 
the high power. Remove fresh strips from the same sunflowers. Place 
the turgid epidermis in a 5% solution of common salt and the flaccid 
one in distilled water. Examine under the high power and explain the 
results. Make a schematic drawing of a stoma when open and when 
closed. 

Experiment 18. Position of stomata and water loss. Select four 
similar leaves of sunflower or, better, of a plant which has stomata only 
on the lower surface. Cover the upper side of one leaf with wax melting 
at a low temperature. Wax the lower surface of the second and both 
surfaces of the third. The fourth leaf is not coated, in order to serve as 
a check. Fix each leaf in a small vial filled with a known amount of 
water, in such fashion that the water lost is supplied through the petiole. 
Determine the loss from each leaf by weighing or measuring the water 



ADJUSTMENT TO WATER 63 

in the vial. Remove the leaves from the vials and premit them to 
dry out, noting the time required in each case. 

80. The influence of physical factors upon transpiration. 
Three factors, humidity, water content, and light, affect trans- 
piration directly. Their influence is seen not only in their ability 
to cause the stomata to open and close, but also in determining 
the rapidity of transpiration when the stomata are open, and, 
indeed, though in much smaller measure, when the latter are 
closed. We have already seen that light causes stomata to open 
in the morning and close at night, thus resulting regularly in less 
transpiration at night than during the day. In addition the 
greater part of the light energy absorbed by the chloroplast, 
usually more than 95%, is converted into heat, and produces 
water loss or, as it is sometimes called, chlorovaporization. A 
marked decrease in humidity or in w r ater content, as well as the 
two acting together, ordinarily causes the stomata to close, 
while subsequent increase tends to open them. It is usually im- 
possible to distinguish clearly between these two factors in con- 
nection with water loss, since they practically always act together. 
For example, the stomata of water plants remain open even when 
the humidity falls to a minimum, and they likewise stay open in a 
saturated atmosphere, even though the water content be low. 
With reference to the amount or rate of transpiration, however, 
each factor has its own effect. Increased humidity checks water 
loss; decreased humidity promotes it. A reduction in water 
content decreases transpiration, and an increase tends to augment 
it. Transpiration is at a maximum when the water content is 
high or excessive and the humidity very low. 

Temperature pressure, altitude, and wind affect transpiration 
only in so far as they change the humidity or, through the latter, 
the water content. These relations have already been discussed, 
and it is only necessary to state again the facts. High air tem- 
peratures increase water loss; low temperatures decrease it. Soil 
temperatures below T freezing decrease transpiration by rendering 
w T ater content less available. High altitudes, i.e., low pressures, 
promote transpiration; low altitudes, i.e., high pressures, reduce 
it. Dry winds increase water loss; moist winds decrease it. 
Apart from their humidity, winds increase transpiration by re- 
moving the increasingly humid air above the leaf and by aiding 
in the movement of the air in the air-spaces and through the stomata. 



64 



PLANT PHYSIOLOGY AND ECOLOGY 



The effects of slope, exposure, and cover upon the amount of water 
loss are exerted through the factors already discussed. 

81. The measurement of transpiration. Of the many methods 
used to determine the amount of water lost from a transpiring 
plant, the most accurate and satisfactory is to weigh the plant 
in its own soil from time to time. This can easily be done in 
the case of plants grown in pots. A vigorous plant, growing 
preferably in a pot 1 or 1 J dcm. in size, should be chosen for 
study. The entire pot is covered with sheet rubber to prevent 




Fig. 17. — Treatment of plants for measuring transpiration. Three are 
rooted plants in pots; the other two are potometers, the one with a 
rooted plant, the other with the root removed. 



the loss of moisture from its surface; or the outside of the pot 
may be waxed, and the top alone covered with rubber. The latter 
is tied closely about the stem, alongside of which a funnel tube 
is placed for the purpose of supplying water and air to the soil. 
The entire apparatus is weighed, and is then weighed again at 
the desired intervals. The plant should be kept under study 
for a week or two, being placed now and then in different condi- 
tions. Weighings are made at such times as to give the water 
loss for day and night, for periods of different length, and the 



ADJUSTMENT TO WATER 65 

total loss for the period of experiment. The plant will usually 
require watering only once or twice during the experiment, but 
the soil should be aerated every day by blowing air through the 
thistle tube. When water is added, the amount must be care- 
fully measured and recorded. For the sake of comparing differ- 
ent plants, and since the leaf surface varies on account of 
growth and the withering of old leaves, it is very desirable to 
express the amount of transpiration upon the basis of a square 
centimeter or decimeter of surface. The total leaf surface is 
found as indicated in Experiment 15; it is usually unnecessary 
to allow for the stem surface. The transpiration for a square 
decimeter of surface is then found by dividing the water loss 
for the time concerned by the number of square decimeters of 
total area. 

82. Measuring transpiration in the field. The measurement 
of transpiration in the field, i.e., of a plant in its own habitat, 
may be made in the same way. The inconvenience is greater 
only because of the need of taking the scales into the field, or 
of bringing the plant into the laboratory, and of seeing that the 
plant runs no risk of being disturbed or destroyed. The plants 
to be studied are carefully dug in the spring, when they are small, 
and transferred to pots of a size that will necessitate little or no 
repotting. The pots are sunken in the holes from which the plants 
are taken. Readings of transpiration may be made as soon 
as the plant is well established. The surface of the pot alone 
is covered with rubber, since the sides are protected by the soil. 
The plant is weighed and watered from time to time in the way 
already described. Care must be taken to free the surface of 
the pot from earth when it is taken from the soil for each weigh- 
ing. The transpiration is likewise expressed in grams per square 
decimeter of leaf surface. It is further very desirable that the 
plants for experiment be located in the stations where physical 
factor readings are taken. This will make it possible to dis- 
cover the causes of the variations in the amount of transpiration 
shown by plants of different habitats. 

Experiment 19. Influence of factors upon the rate of transpiration. 
Select five well-grown sunflower plants as nearly alike as possible. 
Cover the pots with rubber as indicated above and weigh each one. 
Place one plant as a check in a sunny moist place in the plant-house. 
Alongside of it, place two dishes of measured surface, one containing a 



66 



PLANT PHYSIOLOGY AND ECOLOGY 



known amount of water, and the other moist soil of known weight. 
Of the remaining plants, place one in air as dry as possible, one in a cool 
spot, one in a windy situation, and one in darkness. Weigh each at 
the same intervals. Determine the day and night rate of loss, and 
the total loss for each plant, for the water surface, and for the soil surface, 
basing them upon a square decimeter of surface. Explain the results 



. - : " 


i * 

.,< ■ * * 


\ 5^ • . 








"*'\ ■"■ ' 



Fig. 18. — Portable box and balance for measuring transpiration in 

the field. 

in the different situations. Compute the ratio of loss from one square 
decimeter of the check, from the water, and from the soil surface. 



83. The amount of transpiration in plants. The amount of 
water transpired differs for the individual as well as for the species. 
Much of this variation arises from differences between habitats, 
but species of the same habitat differ widely, entirely apart from 
any variation in physical factors. This is due to the fact that 
species differ both in the amount and character of their tran- 
spiring and absorbing surface. The same is true, though to a 
much smaller extent, of plants belonging to the same species. 
Aside from distinguishable differences of structure or surfaces, 
plants of the same species lose water in varying amounts, some- 



ADJUSTMENT TO WATER 



67 



times behaving as though each had an individuality of its own. 
In the classical experiments of Hales (1727), the maximum 
transpiration of a sunflower was found to be 850 gm. in a twelve- 
hour day, and S5 gm. at night. Sachs found also in the sun- 
flower that the leaf surface lost water about half as rapidly as 
it evaporates from a water surface under the same conditions. 




Fig. 19. — A battery of potometers for the ready comparison of the water 
loss of plants of the same or different habitats. 

According to Wiesner, the Indian corn loses per hour for each 
square decimeter of leaf surface 785 mg. in sunlight, 114 mg. 
in diffuse light, and 97 mg. in darkness, while one of the woody 
mallows (Malva arborea) loses respectively 70, 28, and 23 mg. 
It has been found that a plant of Indian corn transpires 14 kg. 
of water during its period of growth (173 days), and a hemp plant 
27 kg. in a growing period of 140 days. Haberlandt has esti- 
mated that the oat plants covering one hectare lost more than 
2,000,000 kg. during their period of growth, while a similar num- 
ber of barley plants transpired more than 1,000,000 kg. The 
transpiration of a large tree reaches an enormous total during 
one summer. Von Hohnel has estimated that a beech tree tran- 



68 PLANT PHYSIOLOGY AND ECOLOGY 

spired on the average 75 kg. daily during the summer, while a 
birch tree possessing about 200,000 leaves lost nearly 400 kg. 
during one hot day. 

84. Relation between transpiration and absorption. It has 
been previously shown that transpiration and absorption are neces- 
sarily reciprocal, or compensatory. The amount of water absorbed 
determines the amount that can be transpired, and, conversely, 
the rate of transpiration reacts forcefully upon absorption. Leaves 
that bear stomata cannot avoid transpiration, but under ordinary 
conditions this is beneficial instead of harmful. The growth of a 
plant depends upon the amount of water absorbed, and the amount 
of nutrient material dissolved in it. Where the water supply is 
sufficient, plants that transpire the most obtain the greatest 
amount of water and salts, and grow the best. Under the usua- 
conditions, the water absorbed by the roots contains about .01% 
of dissolved salts. In other words, a plant must absorb and 
transpire 10 kg. in order to obtain 1 gram of nutrient salts for 
use. In the case of many water plants, the nutrient content is 
much less, and the amount of water that must be transpired is 
correspondingly greater. Thus, while transpiration is an in- 
evitable process for all plants with stomata, it is more significanl 
for some than for others. Its significance depends upon absorpt 
tion, i.e., upon water content. For plants in dry soils, transpira- 
tion is more or less injurious, and it must be prevented in so far 
as possible. Transpiration is beneficial to plants of moist habitats 
because it promotes their growth. This benefit is still more pro- 
nounced in amphibious plants, where the movement of a large 
amount of water through the plant is necessary to its welfare. 

85. Compensation for increased transpiration. From what 
has just been said, it is clear that the effect of increased tran- 
spiration is determined by absorption: the converse is equally true 
though much less important. Such compensation is naturally 
possible only when the water supply is adequate to meet the new 
demand. Consequently, when the water loss is greatly increased^ 
the final adjustment of the plant is determined by the amount of 
water available at the root surface. As indicated, the compensation 
merely results in increased transpiration when the absorption is 
entirely adequate to the demand. When the water supply is 
inadequate, the stomata close. If this condition is but temporary, 
the stomata open as soon as absorption is able to restore the normal 



ADJUSTMENT TO WATER 69 

turgidity. When the drouth is continued, the plant first wilts 
and then dies. Plants often find themselves in conditions where 
the drouth is sufficiently severe to render the temporary closing 
of the stomata more or less ineffective, though not fatally so. 
Such conditions result in modifications of various sorts, most of 
which decrease water loss, though some are for the purpose of 
increasing the water supply. 

86. Details of the adjustment. The water evaporated from 
the cells bordering air spaces is supplied by osmosis from the 
adjacent cells. The latter in turn draw upon cells nearer the 
transporting bundles, until the demand for more water reaches 
the bundles themselves. The vessels of the leaf meet the demand, 
and the water given up is replaced in consequence of an upward 
movement through the bundles of the stem and root. The deficit 
thus caused is met by the movement of water into the root bundle, 
and is consequently passed along until it reaches the root-hairs. 
The last step in the process of replacement is taken by the latter, 
w T hich absorb the necessary amount of available water. As a matter 
of fact, the root-hair is taking in water at the same time that it 
is passing by diffusion into the adjacent cells. 

If the demand for water reaches the root-hair at a time when 
it can not obtain an equivalent amount by absorption, it yields 
some water to the adjacent cell, but not as much as is needed. 
This is due in part to differences of osmotic pressure in the two 
cells, but in a large degree also, it would seem, to the strong attrac- 
tion of protoplasm for water. Apparently as long as this attractive 
power of the protoplasm of the root-hair is met by absorption, 
water is permitted to pass readily into the adjacent cells. But 
in case this affinity is only partially satisfied, the protoplasm of the 
absorbing cell counteracts in a measure the pull exerted by the 
adjacent cell, and insufficient water passes into the latter. As a 
consequence, both cells become less turgid, and, if transpiration 
continues without compensating absorption, finally lose their 
turgidity entirely. A similar readjustment occurs throughout the 
parenchyma of the root, the innermost cell drawing water from 
the bundle. This loss is met by taking water from the leaf-cells 
along the bundles, and the consequent loss of turgidity passes 
throughout the leaf, finally reaching the epidermal cells. The 
latter withdraw water from the guard-cells, which consequently 
close. 



70 PLANT PHYSIOLOGY AND ECOLOGY 

Experiment 20. Pathway of adjustment. From the cross-section of 
leaf and root, and the longisection of the stem, construct a schematic 
drawing in such manner as to show a strip of the cortical tissue of the 
root in contact with the lower end of the bundle, and a segment of the 
leaf in contact with the upper end. Indicate upon the drawing the 
course by which the change due to increased evaporation through a stoma 
reaches the root-hair, and the course of the return adjustment when water 
is lacking for absorption. 



CHAPTER IV 
ADJUSTMENT TO LIGHT 

87. Relation of the plant to light. As the source of energy 
for the food-making activities of the plant, light is scarcely second- 
ary to water in importance. It is unnecessary only in the case 
of molds, mushrooms, etc., which do not make their own food, 
but use that made by other organisms. The primary response 
of green plants to light is the production of chlorophyll by the 
plastid. Under the influence of light, the chloroplast is able to 
decompose carbon dioxide and water to form sugars or elaborated 
foods. A large amount of the light absorbed is converted into 
heat in the plastid, and produces evaporation. The number and 
arrangement of the chloroplasts are determined by the intensity 
of the light. The latter also regulates the movement of the guard- 
cells, causing the stoma to open in strong light and to close in 
darkness. The direction of the light produces a turning or bend- 
ing of the plant, which is termed phototropism. The normal position 
of leaves is due in large measure to the action of light, and the 
same is true of the day and night positions taken by many of 
them. Finally, the form of the leaf is probably due more to light 
than to any other factor. The effects of light may be summarized 
as follows: 

1. Production of chlorophyll. 

2. Decomposition of carbon dioxide and water to form sugar. 

3. Loss of water from the chloroplast. 

4. Changes in the number and position of chloroplasts. 

5. Daily opening and closing of the stomata. 

6. Turning of stems and leaves. 

7. Day and night position of leaves. 

8. Changes in the form and structure of the leaf. 

71 



72 PLANT PHYSIOLOGY AND ECOLOGY 

88. The nature of light stimuli. The stimulatory action of 
light is exerted primarily upon the chloroplast. To this are prob- 
ably to be traced all of the effects just enumerated, though it is 
impossible as yet to establish this connection in the case of the 
turning of stems and leaves, and the periodical changes in the 
position of the latter. In these the effect of the stimulus does 
not appear in the chloroplast, or the cell containing it, but is trans- 
mitted to a more remote part of the plant, where it becomes evi- 
dent. Since such movements occur only in green plants, and 
apparently result in placing the leaves in more favorable posi- 
tions, it seems extremely probable that they are due in the first 
place to the reaction of the chloroplasts to light. 

Stimuli result from a change in the intensity, direction, or 
quality of the light. In nature, the quality of the light is very 
little if any different in various habitats. Even in dense for- 
ests, the diffuse light is white, not green. The direction of the 
light is of little importance, except where the illumination is strongly 
one-sided. This is the rule in horizontal leaves, though the posi- 
tion really results in a difference in intensity. Plants on the 
edges of forests and thickets are often bent toward the sunshine, 
but within a particular habitat such movements are lacking, 
except in the case of a few plants, as the sunflower. The change 
in light intensity necessary to produce a response varies for differ- 
ent species, and it is also influenced by the intensity in which 
the species normally grows. The normal extremes of intensity 
are full sunshine, represented by 1, and a diffuseness of .002, 
i.e., light 500 times weaker than sunlight. The food-making 
activity of the chloroplast is so fully dependent upon the light 
that it is affected by very slight differences of intensity. On 
the contrary, such responses as the movement of the chloroplasts, 
changes in leaf structure, and phototropism are produced only 
by much greater differences. 

89. Measurement of light. Determinations of light intensity 
are made by means of the photometer. This is a tight metal 
box containing a central wheel upon which a strip of photographic 
paper is fastened. The wheel is revolved past an opening 6 mm. 
square, which is closed by means of a slide working closely between 
two flanges. The disk or the wheel is graduated into twenty-five 
parts, which are numbered. A line just beneath the opening 
coincides with the successive lines on the disk, and indicates the 



ADJUSTMENT TO LIGHT 73 

number of the exposure. The movement of the wheel is regu- 
lated by a click. The metal case is made in two parts in such 
way that the bottom may be readily removed and the -trip 
placed in position. The photographic paper used is the kind 
called "solio." The photometer must be filled in the dark room, 
or at night in weak light. A strip 6 mm. wide is cut lengthwise 
from the 8X10 sheet, and I of an inch is cut from one end in 




Fig. 20. — A new form of the simple photometer, showing the numbered 
face and the slit for making exposures. 

order to secure the right length. A crease is carefully made J of 
an inch from each end to prevent the breaking of the paper when 
the cork plug is put in place. The strip is then placed upon the 
wheel, great care being used not to touch the coated surface with 
the fingers. It is fixed in position by using a piece of cork to 
hold the creased ends in the slit of the wheel. The latter is then 
placed in the case, the zero turned until it is opposite the index 
line, and the photometer is ready to be used. 

90. Making a standard. An exposure is made by moving the 
slide quickly, in such a way as to uncover the entire opening. 
Care must be taken not to pull the slide completely out of the 
groove, as it is impossible to replace it with sufficient quickness. 
The length of exposure is determined by means of a watch. A 



74 PLANT PHYSIOLOGY AND ECOLOGY 

stop-watch is especially valuable for this purpose, but a watch 
of the usual type is satisfactory. After a little practice, it is a 
simple task to hold the watch with the one hand against the 
photometer in the other, in such fashion that the slide may be 
moved without taking one's eyes from the second-hand. Imme- 
diately after each exposure, the disk should be turned to the 
next line: this should be made an absolute rule. Except in the 
case of readings made for special purposes, the instrument is 
held with the edge toward the south, with the opening upper- 
most in the usual position of the leaf. 

Standards and ordinary readings are made in practically the 
same manner. There are various kinds of standards, but the 
most satisfactory for ordinary purposes is a temporary or proof 
standard bearing a series of exposures. This is obtained by 
exposing a strip to full sunshine at noon upon a clear day. Expo- 
sures are made for 1, 2, 3, 4, and 5 seconds successively, the great- 
est care being taken to make the time of exposure exact. Fre- 
quently it is advisable to make a second series to serve as a check 
upon the first. After it is completed the standard is removed 
and placed in a light-tight box, such as is used for photographic 
plates. It should be used only in gaslight or in ruby light. The 
former permits much more accurate comparison. When kept 
in a cool place and handled carefully, the standard and the solio 
strips may be preserved for several months without appreciable 
change. Ordinarily, a single standard is sufficient for an entire 
growing period, though it is sometimes desirable to make a new 
one from time to time to serve as a check upon the original. 

91. Making readings. The best practice in making readings 
is to secure a decided tint that falls between the extremes of the 
standard. It is practically impossible to obtain a sunshine equiv- 
alent for very faint tints, and equally difficult to match the very 
deep ones obtained by long exposure. The most satisfactory 
method consequently is to expose until a good tint is secured, 
but one that is not stronger than the 5-second tint of the standard. 
In deep shade, this often requires a long time, and in such places 
it is usually more satisfactory to stop the exposure with a lighter 
tint, approximating the 1- or 2-second exposure of the standard. 
In taking readings, the date, time of day, station, number of 
instrument and of exposure, and the length of the latter in 
seconds, must be carefully recorded. As a rule, readings are 



ADJUSTMENT TO LIGHT 75 

made only on clear days, except where the light values of cloudy 
days are desired for special purposes. After a strip has been 
completely exposed, it is removed in the dark, and a new one 
is put in place. The former is carefully labeled, and dated upon 
the back, and is then filed with the same care as the standard. 

92. Comparison with the standard. The light intensity de- 
noted by each exposure is ascertained by comparing the latter 
with the standard. The strips are placed alongside of each other 
in gaslight of fair strength, and the exposure is moved along 
the standard until the tint that matches it is secured. With a 
little practice this may be readily done. Skill and certainty in 
making the tints match are obtained by comparing the exposed 
strip with the standard a second time. If this is done without 
reference to the first results, the two comparisons serve as a valu- 
able check upon each other. When the proper match is secured 
for a particular exposure, the comparative light intensity is found 
by dividing the length of exposure in seconds by the length for 
the standard tint. Thus, if an exposure in deep shade for 180 
seconds matches the 2-second standard, the light is 90 times 
more diffuse or weaker than the sunlight. The latter is taken as 
unity, and the light intensity of the shade is written in the form .01. 

93. Causes of variation in light intensity. The primary object 
of light readings is to determine the amount of light in various 
habitats, as a basis for explaining the differences shown by the 
plants in them. The real differences between habitats arise from 
the presence or absence of a primary layer, as well as from the 
character of the latter. In striving to measure such differences, 
it is absolutely necessary to avoid errors arising from the con- 
dition of the sky, or from time or place. Clouds have a marked 
effect upon the amount of light, almost invariably by reducing 
it. This difficulty, as already indicated, is entirely eliminated 
by taking readings upon clear days alone, avoiding even those 
when the sky is slightly hazy. 

94. The effect of time. The intensity of the light varies 
throughout the day and the year. The daily maximum occurs 
at noon " sun-time," a point which itself moves back and forth 
through the year. The annual maximum falls on June 22, the 
minimum upon December 22. The daily minimum is reached 
at nightfall, and it lasts until dawn. In both cases the greatest 
light intensity occurs when the sun is at its highest altitude, i.e., 



76 PLANT PHYSIOLOGY AND ECOLOGY 

when the angle that it makes with the surface of the earth is 
greatest, and the lowest when this angle is the least. At equal 
distances from either maximum, e.g., at 8 a.m. and 4 p.m., or 
on March 21 and September 23, the angle is the same. The effect 
of angle upon light intensity is due to the absorption of the light 
rays by the earth's atmosphere. This absorption is greatest 
near the horizon, where their pathway is the longest, and it is 
least at the zenith. In other words, the absorption is greatest 
at sunrise and sunset, least at noonday; greatest in December, 
least in June. Considering the two at the same time, maximum 
sunlight occurs at noon on June 22, minimum sunlight at sun- 
rise or sunset on December 22. 

From the foregoing it is clear that the variation in light due 
to time must be taken into account. In critical investigation, 
it is desirable to compute the error arising in this way, and to elim- 
inate it from the reading. For ordinary purposes, it suffices to 
make readings directly comparable by restricting the times at 
which they are made. In a growing period of six months begin- 
ning March 21 and closing September 23, the noon intensities 
are respectively .98 and .93. 1 Between 9 a.m. and 3 p.m. during 
this period, the range would be from .82 to .98, a difference that 
is practically negligible. Hence, in all ordinary study, the value 
of readings taken in full sunlight between 9 a.m. and 3 p.m. from 
March 21 to September 23 may be regarded as unity. Conse- 
quently, readings made in shady habitats during the same period 
may be compared with them directly without making allow- 
ance for the slight error. Naturally, when readings are taken 
simultaneously, no error exists, and the same is practically true 
of readings made between 10 a.m. and 2 p.m. 

95. The effect of altitude. Altitude affects the amount of 
light by decreasing the distance which the rays must travel through 
the atmosphere, and thus decreasing the absorption. This influ- 
ence is much smaller than has been commonly supposed. It is 
estimated that 20% of a light ray is absorbed before it reaches 
sea-level. At the top of Pike's Peak (4267 meters high), the 
absorption is 11%. In the one case the light is 80% of that 
which enters the atmosphere, in the other 89%. Consequently 
the maximum intensity at sea-level and at the summit of Pike's 
Peak is .98 and 1.09 respectively. The difference between them 
1 Research Methods, 57. 



ADJUSTMENT TO LIGHT 77 

is altogether too small to have any important effect. The amount 
of light received by a slope differs from that received by a level 
area of the same extent. Since the leaf position is the same in 
both cases, this difference is of no significance. 

Experiment ax. Measuring light intensity. Make a standard be- 
tween 10 a.m. and 2 p.m. on a sunny day, and then take a reading in 
the open, one in the plant-house, and one in each shade-tent. Repeat 
the series at the same time upon a cloudy day. Remove the strip, 
and find the value of each reading. Is the ratio of intensity in the 
various places the same for cloudy and for sunny days? 

96. Reception and absorption of light. Plants possess no 
special structures for the reception of light. The latter falls alike 
upon all aerial parts, the characteristic form of the leaf being 
chiefly to increase the surface. The epidermal cells, which re- 
ceive the light, merely transmit it, and the stimulus is first felt 
in the chlorenchym beneath. The general effect of the epidermis 
is to reduce the amount of light that enters the leaf. This is due 
partly to reflection and partly to absorption. Leaves with a 
smooth or shining cuticle reflect the light, w T hile a thick cuticle 
or a dense coating of hairs absorbs much of it. In some cases 
the shape of the epidermal cells is such that the light passing 
through the cuticle is more or less concentrated before it reaches 
the chlorenchym. This is a rare occurrence, however, and of 
little importance. The rule is that by far the greater amount 
of the light that enters the leaf passes through the epidermis and 
is absorbed by the chlorenchym. Some light passes entirely 
through the leaf, but ordinarily this is slight. Thick or fleshy 
leaves absorb practically all the light that falls upon them. Thin 
leaves placed in sunshine transmit considerable light, but it must 
be remembered that such leaves are usually confined to shady 
habitats, where the light is very diffuse and the absorption rela- 
tively complete. 

97. The amount absorbed. The amount of light actually used 
by the leaf is ascertained by determining the amount that passes 
through the epidermis, and by taking from this that which passes 
through the entire leaf. The amount of light available for the 
chlorenchym is measured by stripping a piece of epidermis from 
the leaf. This is placed over the opening of the photometer, and 
an exposure made. After the strip is removed, another exposure 



78 PLANT PHYSIOLOGY AND ECOLOGY 

is made to obtain the light intensity at the time. The epidermis 
print, as it is called, is compared with the standard, using the 
second exposure as a check upon the latter. A leaf print is made 
in similar fashion and at the same time by placing the leaf closely 
over the opening. The value of the epidermis print less that of 




Fig. 21. — A leaf print of the sun and shade leaves of four species which 
show both sun and shade forms or varieties. In each the shade leaf 
is the darker. 

the leaf print is a measure of the absorption of the chlorenchym, i.e., 
of that part of the leaf sensitive to light. In the sunflower the 
value of the epidermis print is .1 approximately, that of the leaf 
print .003, and the light absorbed by the leaf is consequently .097. 

Experiment 22. Epidermis and leaf prints. Strip from the upper 
surface of a sunflower leaf a piece of epidermis sufficient to cover the 
opening of the photometer. Make an exposure beneath the strip, and 
with the same or a similar leaf make a leaf print. Take a third reading 
to obtain the full light intensity. Compare the results with the standard, 
and determine the amount of light screened out by the epidermis, 
absorbed by the chlorenchym, and transmitted through the entire leaf. 



ADJUSTMENT TO LKillT 



79 



98. The production of chlorophyll. The primary response of 
the plant to light is the production of chlorophyll. This response 
does not occur in plants, such as the bacteria, mushrooms, broom- 
rape, etc., in which the power to make chlorophyll has been lost 
in consequence of parasitic or saprophytic habits. On the other 
hand, a few plants, conifers, ferns, and cacti, are able to make 
chlorophyll in darkness, though it probably cannot be formed 
continuously under such conditions. The rule is that plants with 
plastids produce chlorophyll only in the light. Conversely, their 
chlorophyll disappears in darkness. The light intensity necessary 
for the production of chlorophyll varies in different species. This 
pigment is formed in shaded habitats with light values as low as 
.001, even though very few flowering plants can function at all 
under such conditions. For the majority of plants, sunshine 
presents the best conditions for the formation of chlorophyll. If 
the sunlight becomes too concentrated, it tends to decompose the 
chlorophyll more rapidly than it can be built up. Although this 
rarely occurs in nature, chlorophyll undergoes constant decom- 
position by light, and it persists only because it is built up at the 
same time. 

99. The nature of chlorophyll. Chlorophyll is a complex 
pigment, or a mixture of several pigments. It is produced by 



b c D 

, 10 '. 20 30 



I 



40 50 

I I I 



90 100 




Fig. 22. — Absorption spectrum of chlorophyll removed from the leaf by 
solution in alcohol. The letters B-G indicate the position of Fraun- 
hofer's lines. The groups of dark lines from the red to the violet end 
indicate the seven absorption bands of chlorophyll, i.e., those parts 
of the spectrum in which the light is absorbed by the chlorophyll. 
(After Sachs.) 



specialized bits of protoplasm, the plastids. When the necessary 
conditions are met, i.e., the presence of light, water, carbon dioxide, 
and a trace of iron salts, the plastids of active tissues are always 
green, i.e., they are chloroplasts. The latter lose their color in 
the dark, or, ordinarily, when carbon dioxide is absent. The 
pigment is broken down in ripening fruits and in flowers, and also 



80 



PLANT PHYSIOLOGY AND ECOLOGY 



in autumn leaves. Chlorophyll is a product of protoplasmic ac- 
tivity, formed during the complex processes of nutrition. Appa- 
rently it is not a part of the protoplasm, but is held in solution 
in it. It is produced most actively in white light, but it has been 
found that the yellow rays are the most active 
in its formation. The function of chlorophyll is 
not definitely known. The view commonly ac- 
cepted is that it absorbs certain rays of light, 
particularly the red and the blue, converting them 
into a form of energy capable of decomposing 
carbon dioxide and water. It has also been sug- 
gested that the rays absorbed are harmful to the 
activity of the plastid, and are eliminated in order 
to promote the action of the other rays. What- 
ever the exact control exerted by the chlorophyll 
may be, there is no question of its necessity for 
the formation of elabo- 
rated food by the decom- 
position of carbon dioxide 
and water. 

ioo. The influence of 
darkness. Disregarding 
the exceptions already 
noted, seeds germinated 
in the dark produce plants 
that are whitish or yellow- 
ish in color. The same 
result occurs in green 
plants which are placed 
in darkness. An exam- 
ination of the plastids 
shows that they are more 
or less yellow in color, due 
to the presence of a pig- 
ment called etiolin. The 
latter is apparently con- 
vertible into chlorophyll, since a yellowish plastid becomes green 
upon exposure to light. Plastids which occur regularly in the 
dark, viz., leucoplasts, are also capable of turning green in light, 
as is well known in the case of the potato tuber, but the existence 




Fig. 23. — Potato-plants grown in darkness 
(A) and in light (JB). The elongation of 
corresponding internodes may be com- 
pared by reference to the figures. (After 
Pfeffer.) 



ADJUSTMENT TO LIGHT 81 

of an etiolin-like substance in them is unknown. The yellowish 
or colorless plants produced by darkness are said to be etiolated. 
Etiolation affects not only the color of the plant, but its form 
and structure as well. Stems usually become thin and elongated, 
and their branching is reduced. The size and number of the leaves 
are decreased, and the latter are often reduced to mere scales. 
There is a corresponding reduction in their structure, the dis- 
tinction between palisade and sponge tissues disappearing as a 
rule. 

Experiment 23. Influence of light and darkness. Plant sunflower 
seeds in four small pots. Place two pots in the dark, and leave two in 
the light. After the seedlings have grown to a few inches, exchange a. 
green plant and a colorless one. After a few days note the behavior 
of the chlorophyll. Make a drawing of the plant that has been con- 
stantly in the dark, and of the one kept in the light. Make a cross- 
section of the stem and leaf of each, and illustrate the important differ- 
ences by a drawing. 

10 1. Photosynthesis. The decomposition of carbon dioxide 
and water by the action of light, and the combination of the products 
into sugars, is termed photosynthesis. It is the special duty of 
the chloroplast, and can take place only in light. It is most 
active in sunlight, decreasing with a decrease of light intensity, 
though at a varying rate for different species. For flowering 
plants, photosynthesis becomes practically impossible, even for 
shade forms, when the light intensity reaches .001. In the case 
of many sun plants, the minimum is reached long before this, 
depending upon the stability of the species concerned. In nature 
photosynthesis is always due to the presence of white light. By 
experiment it may be shown that certain colors of the spectrum 
are more active than others in this process. It is found that the 
greatest activity occurs in orange-red, somewhat less activity in 
the blue, and little or none in other portions. These regions are 
those in which the absorption of light by the chlorophyll is greatest,, 
a fact which gives support to the view that it is this absorption 
which furnishes the energy necessary for photosynthesis. 

102. Absorption and diffusion of carbon dioxide. Photo- 
synthesis depends, moreover, upon the presence of the crude 
materials with which it is concerned, viz., carbon dioxide and 
water. We have already seen how the water absorbed by the roots 



82 PLANT PHYSIOLOGY AND ECOLOGY 

reaches the leaves, and consequently the chloroplasts. The carbon 
dioxide required is absorbed by the leaves through the stomata. 
With the air it enters the chamber below the guard-cells, and then 
finds its way throughout the leaf by means of the air spaces. 
The degree of photosynthetic activity depends in a large measure 
upon the rapidity with which carbon dioxide enters the leaf and 
diffuses through it. This is due to the fact that, while the supply 
of this gas is practically unlimited, the amount actually present 
in the air is very small, usually about 0.04% by weight. In con- 
sequence aeration, i.e., the movement of gases into and through the 
leaf, becomes a very important function of the latter. It depends 
in the first place upon the number of the stomata, the size of 
their pores, and the length of time they are open. The advantage 
of having the stomata open in the light thus becomes evident, as 
well as that of placing them chiefly upon the lower surface, or of 
sinking them, so that the chances of water loss closing them are 
reduced. Ease of diffusion within the leaf depends upon the 
thickness of the leaf and the number and size of the air spaces. 
The rapidity of diffusion is chiefly determined by the rate at which 
the carbon dioxide is being used. Thus, active sun leaves absorb 
and use much more carbon dioxide than the less active shade leaves, 
although the latter regularly possess larger air spaces. Gases 
penetrate the walls of the chlorenchym cells only in solution. This 
is brought about by the cell-sap which fills the wall, and in 
consequence carbon dioxide and water reach the chloroplast 
together. 

103. Chemical changes during photosynthesis. The precise 
changes that take place during photosynthesis are in doubt. It 
is definitely known that the carbon dioxide and water are broken 
down, and that oxygen is set free and escapes from the leaf. In 
fact, the evolution of oxygen from the plant is the usual test of 
photosynthetic activity. The first visible product of the latter 
is the starch formed in the chloroplast. It is evident, however, 
that a number of changes must occur before starch can appear. 
The number and nature of these changes are not known with 
any degree of certainty. The formation of grape sugar, or glu- 
cose, precedes that of starch, and it is probable that glucose is 
formed from a simpler carbohydrate, formaldehyde. The prob- 
able stages in photosynthesis may be indicated by the follow- 
ing series of formulae. 



ADJUSTMENT TO LIGHT 83 

(1) C0 2 + H 2 = CH 2 3 , carbonic acid; 

(2) CH 2 3 = CH 2 + 2 , formaldehyde and free oxygen; 

(3) 6CH 2 = C 6 H 12 6 , glucose; 

(4) C G H 12 O G -H 2 O = C 6 H 10 O5, starch. 

Experiment 24. Dependence of photosynthesis upon aeration and light. 
Place two vigorous sunflower plants in darkness for twenty-four hours 
in order to bring about the disappearance of the starch. Use wax or 
vaseline to coat the upper surface of one leaf, the lower surface of a 
second, both surfaces of a third, and allow a fourth leaf to remain un- 
coated. Place this plant in full sunshine, leaving the other in the dark. 
This experiment should be started as early in the morning as convenient. 
During the afternoon of the same day, cut cross-sections from each of 
the four leaves of the plant in sunlight, and from one of the leaves in 
darkness. Note the relative amount of starch in each as well as its 
distribution. 

104. Measurement of photosynthesis. The intensity of photo- 
synthetic activity in different plants, or in the same plant under 
different conditions, is best measured by the quantity of starch 
produced. This is not an exact measure, since much of the glu- 
cose formed is removed at once instead of being converted into 
starch. However, since the amount of glucose produced can not 
be measured, the degree of activity can only be based upon the 
starch formation. This can be done by treating entire leaves 
with iodin, as in the method employed by Sachs. 1 A more accu- 
rate method is to cut sections of the leaves to be studied, and 
to count the starch grains, and in some cases the number of plas- 
tids. The count is made for a segment 100 pt wide across the 
entire section. Two such segments are counted in different parts 
of the leaf, and the result is multiplied by five to give the num- 
ber of starch grains for a unit segment 1 mm. in width. 

Experiment 25. Relation of photosynthesis to sun and shade. Remove 
the starch from two sunflowers by keeping them in darkness for twenty- 
four hours. Place one plant in sunshine and one in a shade-tent of known 
light intensity. After six or eight hours, cut sections of a leaf from 
each, and determine the starch content for a unit segment. Compare 
the results in the two leaves with respect to the light intensity for each. 

105. Translocation. A part of the glucose made by the chlo- 
roplasts is used by the leaf itself, while a large amount is carried 

1 Research Methods, 137. 



84 PLANT PHYSIOLOGY AND ECOLOGY 

away to the stem, flowers, and roots. Starch appears in the chlo- 
rop lasts only when the glucose is formed more rapidly than it is re- 
moved. This is usually the case in the sunlight, and starch accumu- 
lates throughout the day. When photosynthesis ceases at night, 
the starch in the plastid is converted into glucose by the chemical 
addition of a molecule of water to each molecule of starch, and the 
glucose is transported in solution to the different parts of the plant. 
The removal, or translocation, of the glucose normally takes place 
during day and night, but, since this substance is usually made about 
ten times faster than it can be removed, translocation is most active 
at night. This movement of the soluble food is largely caused 
by differences in osmotic pressure between the various cells. Its 
direction is determined by the greatest use, the movement in 
general being toward the organ or tissue that is most actively 
assimilating or storing the food. In the chlorenchym as well 
as in the parenchyma of the stem and root, the glucose moves 
by ordinary diffusion. In its passage from the leaves to the 
other organs of the plants, it follows a more definite pathway. 
This is regularly constituted by the sieve tissues of the fibro- 
vascular bundle, though it is not impossible that glucose should 
reach the root by diffusion through the parenchyma of the stem. 
This is a slow process, however, and by far the greater amount 
of glucose must reach the various organs by way of the sieve 
tissue. Thus, the fibrovascular bundles constitute a complete 
system for the transport of the foods made by the leaves, as well 
as for that of the water and solutes absorbed by the roots. 

Experiment 26. Translocation. Select a vigorous sunflower that has 
been in the sunshine all day. Cut a leaf from it and put the leaf away 
in a moist chamber. Sever the midrib or primary veins of a second 
leaf, cutting through almost to the upper epidermis. Early the next 
morning, remove the severed leaf and an uninjured one from the plant. 
Cut cross-sections of the three leaves, and explain the differences in 
starch content. 

106. Storage of food material. The storage of glucose in the 
form of starch may take place in practically any part of the plant. 
Except for the temporary formation of starch in the chloroplasts, 
storage in the green leaf is rare. It occurs very often in seed- 
leaves, and frequently in seeds, stems, and roots. In all cases 
the method by which the material is stored is the same. It is 



ADJUSTMENT TO LIGHT 85 

accomplished by the leucoplasts, and hence can occur only in 
living cells. The sugar solution is taken up by the leucoplasts, 

which remove by chemical action a molecule of water from each 
molecule of the glucose, and deposit the resulting starch in their 
own substance. This deposition of starch takes place rapidly 
at night, and slowly, if at all. by day. In consequence, the starch 
grains of storage tissues regularly show layers corresponding to 
this periodical activity. As the starch grain grows, the leuco- 
plast is stretched thinner and thinner until it becomes invisible. 
It never disappears, however, since, at the time when the reserve 
material is to be used, it must dissolve the starch by again com- 
bining water with it to form sugar. 

Starch is stored to serve as food material for starting new 
growth and continuing it until a constant supply of food can 
be furnished by the new leaves. In the seed, it nourishes the 
young plant until the latter strikes root and carries its leaves 
into the light and air. The stored material of root or rootstalk 
gives a new shoot, while that of the woody stems enables the 
buds to burst and to send out leaves and flowers. In seeds the 
reserve material is stored in the endosperm about the embryo, 
as in the grasses, or it is stored in the leaves of the embryo itself, 
e.g., in the bean, pea, etc. In perennial plants, storage often 
occurs in the stem. In herbaceous perennials, the underground 
stem, rootstalk, tuber, bulb, or corm serves this purpose; in trees 
and shrubs, the pith and medullary rays of the woody stem 
itself are used for storage. True roots serve more rarely as store- 
houses, but in certain biennial and perennial herbs they have this 
function. 

Experiment 27. Storage tissues. Make cross-sections of a wheat 
kernel, a bean, a woody stem, a rootstalk or tuber, and a root. Stain 
lightly with iodin, and look for leucoplasts. Make a diagram of each 
section showing the extent and the location of the storage tissues. 

107. Influence of light upon the number and position of 
chloroplasts. Only a small amount, usually less than 5%, of the 
light that reaches the chloroplast is used in photosynthesis. The 
remainder is converted into heat, and produces a vaporization of 
the water in the plastid. The effect is less pronounced in the 
shade owing to the lower light intensity. In consequence, chloro- 
plasts arrange themselves in the sunshine in line with the light 



86 



PLANT PHYSIOLOGY AND ECOLOGY 



rays, so that they screen each other from the full effect of the 
light, and thus reduce the amount of water they lose. This arrange- 
ment is not a necessary protection against over-illumination, for 
the chloroplasts of the guard-cells receive stronger light without 
serious harm, but it appears to be a device to prevent injurious 
water loss from the chloroplasts while they are active. In the 




Fig. 24. — Position of chloroplasts in sun leaf (1) of Allionia linearis, in a 
leaf of the shade form (2), and in one found in very deep shade (3). 

shade the danger of excessive water loss is slight, while the need 
of obtaining all the light possible is imperative. Accordingly, the 
plastids arrange themselves at right angles to the light ray, thus 
increasing the surface for light absorption. This arrangement of 
the plastids is found not only in sun and shade leaves respectively, 
but it is also typical of horizontal leaves. In fact, the latter 



ADJUSTMENT TO LIGHT 87 

owe the differentiation of the chlorenchym into palisade and 
sponge tissues to this fact. The upper part of the leaf receives 
full sunlight, and the plastids place themselves in line with the 
light rays. The lower portion receives only the light that is not 
absorbed by the upper. It is just as truly shaded as a leaf growing 
in a forest, and the chloroplasts spread out in such fashion as 
to receive as much light as possible. This arrangement is the 
rule, but it is not absolute, owing to local modifications due to the 
position of air passages, and the necessity of diffusion from cell 
to cell. The position of the plastids influences the form of the 
cells, so that, while both palisade and sponge cells are more or 
less elongated, the palisade cells are placed at right angles to the 
leaf surface, and the sponge cells are parallel to it. This relation 
is doubtless connected with the fact that sun leaves are generally 
narrower and thicker in comparison with shade leaves, which are 
thinner and broader. The influence of light upon the form and 
structure of the leaf is considered under Adaptation. 

Experiment 28. Arrangement of chloroplasts. Fix one of a young 
pair of leaves of a sunflower with the lower surface uppermost and the 
other in an erect position. After they have become fully grown, cut 
cross-sections of each, and compare with the cross-section of a leaf 
in the normal position. "Slake drawings of changes in the number and 
arrangement of the chloroplasts, as well as in the shape of the cells. 

108. Movement of stems and leaves. Through the long- 
continued action of light, the stems and leaves of plants have 
acquired certain habits of growth or position. A change in the 
normal relation results in an attempt upon the part of the plant 
to assume a corresponding position. Stems usually place them- 
selves in line with the light rays, while leaves tend to stand at 
right angles to them. If the direction of the light is changed, either 
by shutting it off from all sides but one, or by changing the posi- 
tion of the plant, the stem tries to bend in such a way as to keep 
the same position with relation to the light. Leaves when in- 
verted or otherwise changed from the normal position twist and 
turn in an endeavor to resume it. In somewhat similar fashion, 
roots, which are organs habituated to the absence of light, in 
some plants at least bend away from the light, if they are free to 
do so. Finally, light produces certain continuous periodical 
changes in the position of the leaf. 



88 



PLANT PHYSIOLOGY AND ECOLOGY 



109. Phototropism. Seedlings exposed to light from one side 
alone regularly bend in the direction of the light. This reaction 
is perceptible even with low light intensities. It occurs in adult 
plants as well as in young ones, but it is usually more noticeable 
in the latter. In most cases the stimulus is received by the tip 
of the stem, though in some plants the cotyledons are the part 
stimulated. In either event the stimulus is transmitted to the 
zone in which growth is most rapid, and the response first 



'C~: ■ ; 










k -- - ' 


' .1 V « 



Fig. 25. — Rosettes of a saxifrage, Teleonyx jamesii, growing under an 
overhanging rock. The leaves are turned squarely to the north. 



becomes evident there. Stems that bend toward the light are 
termed prophototropic, while roots and those stems that bend from 
the light are spoken of as aphototropic. Horizontal leaves endeavor 
to maintain a position at right angles to the incident light, and are 
consequently termed diaphototropic. All phototropic movements 
have a common purpose in that they serve to place and keep the 
leaf surfaces in the most favorable position with respect to light. 
A few plants such as the sunflower often respond to the direct rays 
of the sun by turning the crown in the direction of the latter. 
These plants may be termed heliotropic. The leaves of certain 
plants, chiefly leguminous ones, seem to possess a similar property. 



ADJUSTMENT TO LIGHT 89 

Experiment 29. Phototropic movements. Select four sunflower 
seedlings in which the plumules have not yet unfolded. Remove the 
cotyledons from one. Wrap the stem of another with black paper or 
tinfoil from the base to the cotyledons, and wrap the tip of the stem 
of the third in similar fashion. Use a thread moistened with ink to 
mark the upper part of the stem of the fourth at 2 mm. intervals. Place 
the plants in a light-tight box at equal distances from a small hole in 
one end. Remove them after a few days, and explain the results noted 
in each. 

no. Nyctotropism. Many plants change the position of their 
leaves at night. This is particularly true of plants with compound 
leaves, such as the bean, clover, oxalis, sensitive plant, and others. 
The leaflets of these plants close at night, and the entire leaf often 
changes its position, usually by drooping. This phenomenon is 
called nyctotropism. It is thought to be due to the influence of 
light, but it seems that temperature plays at least as active a part 
in bringing it about. The movement occurs at night and morning, 
when the changes in light intensity are the most rapid, but at this 
time the changes in temperature are marked also. The opening 
and closing of many flowers takes place at these times also. These 
have likewise been supposed to be influenced by light, but it is 
now known that they are due to heat. 

Experiment 30. Nyctotropic movements. Grow sensitive plants, 
beans, clover, and oxalis. Compare the position of the leaves at night 
and in the daytime. Change the plants from sunlight to darkness, and 
the reverse, and note the results. 



CHAPTER V 
ADJUSTMENT TO TEMPERATURE 

in. Relations of plants to temperature. In its action upon 
plants, temperature is like water, but unlike light in that it has 
more or less to do with nearly every function. It is not only a 
necessary condition for the chemical changes that occur every- 
where in the protoplasm, but it also furnishes the energy for some 
of them. As a stimulus it differs from both water and light, since 
the responses to it are not localized in a particular organ, but are 
found throughout the living tissues. It has, moreover, no forma- 
tive effect upon the plant, i.e., it cannot change its form or struc- 
ture, except in so far as growth has a bearing upon these. As in 
the case of all factors, habitat plays a very important part in deter- 
mining the influence of temperature upon each species. The 
latter has been accustomed for countless generations to certain 
extremes of heat and cold, as well as to certain seasonal sums. 
Temperatures beyond these extremes check the plant's activity, 
and the same is true of a deficiency in the total heat available 
during the growing period. The effect of habit is well shown 
by many seeds and spores, which are merely plantlets securely 
protected against drouth. These have accommodated themselves 
to a long period of cold, in consequence of which it is often impos- 
sible to cause them to germinate without subjecting them to 
cold, either naturally or artificially. 

Temperature is directly concerned, either as a necessary con- 
dition or as energy, with every function of protoplasm. In a large 
measure, moreover, it controls the movement of the protoplasm. 
It is especially requisite in all the changes that have to do with 
the assimilation of elaborated food, and in a general way, at least, 
may be said to control growth. Under certain conditions, plants 
show changes of position which are caused by heat and may be 

90 



ADJUSTMENT TO TEMPERATURE 91 

termed thermotropic. The opening and closing of flowers and 
flower heads are due to temperature, and to some degree doubt- 
less the drooping of leaves at night and the movements of some 
fruits arise from the same cause. While all functions, absorption, 
photosynthesis, etc., owe much to temperature, certain ones, 
digestion, respiration, growth, and reproduction, are particularly 
dependent upon it. This is likewise true of the germination of 
the seed, which combines the first three functions. In connec- 
tion with germination, temperature has a profound effect upon 
ecesis, or acclimatization, and consequently upon the develop- 
ment of vegetation. 

112. The measurement of temperature. With the aid of the 
thermometer, the determination of temperature becomes one of 
the simplest of tasks. Since temperature is an extremely vari- 
able factor, it is often convenient to use automatic thermometers, 
or thermographs. When thermographs are not available, their 
lack may be partially met by maximum-minimum thermometers, 
which are used to record the extreme temperatures during day 
and night. In all ordinary work, however, the use of the simple 
thermometer in connection with simultaneous readings gives sat- 
isfactory results. The thermometers used should be standard or 
standardized instruments reading accurately to one degree, or, 
better, to one fifth of a degree. As these are both delicate and 
expensive, they must be used with great care, particularly in the 
field. In making readings of air temperatures, precautions are 
necessary to expose the bulb to the full effect of wind or sun, 
when these are present, and to keep it away from the hand or 
person. Moreover, in taking readings in rapid succession, the 
instrument must be left in each position until the mercury becomes 
stationary. In some cases, as when the wind blows fitfully, the 
mercury rises and falls constantly. In simultaneous readings 
this fluctuation may be ignored, and the precise position at the 
time of the signal is the one taken. Otherwise the mean of the 
fluctuation is taken as the proper reading. Readings at the sur- 
face of the soil are made in the same way. In all cases the bulb 
must touch the surface and not lie separated from it by a stratum 
of air. The temperatures are different for the soil surface and 
for dead or living vegetation lying upon it. Consequently this 
fact must be taken into account in comparative readings made 
in different stations. 



92 PLANT PHYSIOLOGY AND ECOLOGY 

113. Soil temperatures are determined by means of the instru- 
ments used in the air, or by means of thermometers specially 
constructed for the purpose. The latter are thermometers pos- 
sessing a very long tube, the whole instrument being encased in a 
wooden jacket for protection. The soil thermometer is placed 
in the ground with the bulb at the desired depth. The scale is 
then above the surface, where it may be read directly. Such 
thermometers, however, are relatively expensive. Their placing 
requires considerable time and trouble, and their use is restricted 
to the few places where they would be entirely free from dis- 
turbance. Soil temperatures are relatively constant, especially 
at depths greater than three or four decimeters. In consequence, 
frequent readings of them are unnecessary, and the simplest 
thermometer yields satisfactory results. Soil temperatures are 
usually measured in connection with water content, the hole 
from which the soil sample is taken serving for the temperature 
reading. Unless the latter is made as soon as the sample is re- 
moved, the hole is stopped with a cork to prevent any change 
in temperature. In making the reading, the thermometer is 
lowered carefully into the hole, the cork is put in place, and the 
instrument allowed to remain for several minutes to make sure 
that the mercury indicates the proper temperature. In the case 
of holes deeper than the thermometer is long, a cord is tied to 
the top of the latter to assist in raising or lowering it. In obtain- 
ing the temperature, the thermometer is not lifted out of the 
hole, but is raised in the latter until the upper end of the column 
can be read. With a little practice, this can be done very easily 
before the column begins to rise or fall. When several soil read- 
ings are made in one station on the same day, the use of the cork 
as a stopper makes it possible to use the same hole without appre- 
ciable error. If readings are made on different days, it is desir- 
able to bore a new hole. 

114. Plant temperatures. Unlike the warm-blooded animals, 
plants do not possess temperatures that are independent of the 
surrounding medium. Certain plant activities, notably respira- 
tion, bring about an evolution of heat, but in the vast majority 
of cases this is so slight as to have no appreciable effect upon the 
plant. As a consequence, plants and plant parts tend to assume 
the temperature of the medium, viz., air, soil, or water, in which 
they are found. In other words, they behave essentially as non- 



ADJUSTMENT TO TEMPERATURE 93 

living matter does. The temperature of the plant, especially 
in the case of aerial parts, is not always the same as that of the 
medium, for several important reasons. The temperature of the 
plant responds more or less slowly to that of the air, and in the 
case of a sudden change it is for a time greater or less than that 
of the air. In fact, the temperatures of the plant behave more 
like those of the soil than of the air. The plant lags in the change 
in proportion to its mass and to the character of its surface. 
Transpiration probably has some effect upon the leaf surface. 
This effect does not seem to be great, since the leaves of rosettes 
often have a temperature as high as that of the surface of the 
soil upon which they are found, and, in mountain regions at least, 
the temperature of sun leaves is higher than that of the surround- 
ing air. 

In taking internal temperatures of plants it is desirable to use 
a thermometer with a bulb as small as possible. In the case of thick 
stems and roots or other fleshy parts, a slit is cut or a hole is 
made with a cork-borer, and the entire bulb sunken in the tissue. 
Temperature readings are obtained less easily in the case of ordinary 
leaves. Fairly satisfactory results have been secured by rolling 
the leaf while in position tightly about the thermometer bulb. 
When it is possible, the thermometers are left in place, in order 
that readings may be made at various times, thus showing the 
connection between the temperature of the plant and of the 
medium. 

Experiment 31. Temperatures of plant and habitat. In addition to 
the temperature readings in the field outlined in Experiment 5, make 
simultaneous readings of air, soil, and plant temperatures. This can be 
done most conveniently in the plant-house, although it is desirable to 
make similar observations out of doors in the spring. 1 A plant with a 
fleshy root, such as the beet, is the most satisfactory. Bore a hole of 
the proper size in the root, and sink the thermometer bulb well below 
the surface. Roll the tip of a leaf closely about the bulb of a second 
thermometer, folding in the edge of the leaf in such a manner as to 
cover the bulb entirely. Hang one thermometer in the air alongside 
the leaves but in the sun, and place another in position for making soil 
readings. Finally sink another bulb in dead or inactive tissue, such 
as that of an apple or potato. Read the five thermometers as nearly 
simultaneously as possible at various times during the day. The best 
results are obtained by making readings early in the morning, at noon, 

1 Research Methods, 88. 



94 PLANT PHYSIOLOGY AND ECOLOGY 

in the evening, and at night. Compare and explain the temperatures 
obtained. 

115. Variations of temperature. As in the case of light, there 
is a daily and an annual fluctuation in temperature. The amount 
of heat received depends upon the angle of the sun's rays and 
their consequent absorption. The actual temperatures at the 
surface of the earth are greatly modified by radiation, conduction, 
and convection. In consequence the maximum daily temperature 
does not occur at noon " sun-time," as in the case of light, but 
several hours later, usually about 4 p. m. The minimum is not 
reached at nightfall, but just before sunrise upon the following 
morning. The maximum temperatures for the year do not occur 
at the June solstice, but a month or two later. Similarly, the 
minimum falls a month or more after the December solstice. 

The variation of temperature with latitude, and altitude is 
well known. Northern latitudes receive the sun's rays at a greater 
angle than southern ones, and the absorption of heat by the atmos- 
phere is correspondingly greater. In so far as absorption is con- 
cerned, high mountains receive more heat than lowlands. The 
loss by radiation, however, is so much greater that mountain 
regions are uniformly colder than plains or lowlands lying on the 
same parallel. This is due to the extreme rarity of the air, which 
allows heat to pass through it readily. Although the air on moun- 
tain tops is colder than that of the plains, the surface temperature 
of the soil is often considerably higher. This difference, however, 
is far overbalanced by the rapid radiation at night. Temperature 
also varies with the slope. This is due to the fact that a square 
decimeter of sunshine covers this amount of surface only when the 
rays strike the latter at right angles. As the angle diminishes, the 
rays are spread over more and more surface until at 10° a square 
decimeter receives but 17% as much heat as at 90°. It has already 
been indicated that this has slight importance, owing to the fact 
that stems and leaves have the same position upon a slope that 
they do upon the level. Furthermore, temperature differs at 
various levels in the air and the soil. Air and soil temperatures 
naturally affect each other. The highest temperatures are usually 
found between the two, i.e., at the surface of the soil. In both 
directions the temperature rapidly decreases. In the air this 
is due to the fact that radiation becomes imperceptible a short 
distance above the ground, while the influence of the wind be- 



ADJUSTMENT TO TEMPERATURE 95 

comes more and more noticeable. Heat penetrates the soil slowly, 
either on account of poor conductivity or because of the capacity 
of the water content for latent heat. The temperature of the 
soil decreases rapidly in the first few decimeters, and at the depth 
of a meter often remains constant throughout the growing season. 
The air is ordinarily warmer in the daytime than the soil, especially 
on sunny days. It loses heat more easily, however, and after a 
sudden change or at night the soil is for a time warmer than the 
air. On account of the high specific heat of water, dry soils are 
regularly warmer than wet ones. 

116. The influence of other factors. Clouds and winds are 
among the most important factors that modify temperature. 
During the daytime clouds decrease temperature by absorbing 
the sun's rays, while at night the effect is exactly opposite, on 
account of the hindrance they offer to radiation. Winds raise 
temperatures when they blow from a warmer region, and lower 
them when they come from a cooler one. Humidity acts after 
the manner of clouds, a humid air increasing the amount of heat 
absorbed. Soil temperatures are affected not only by the amount 
of water present, but by the character of the soil. Sand and 
gravel are more easily heated than clay and loam, and the air 
above them is also warmer because they lose heat more rapid ly 
by radiation. The effect of exposure is closely connected with 
slope. Slopes that face the south and west receive the most sun- 
shine, and are regularly warmer than north and east slopes. Cover, 
whether dead or alive, reduces day temperatures by screening out 
the sun's rays, and increases night temperatures by retarding 
radiation. 

117. Favorable and unfavorable temperatures. It is hardly 
necessary to point out that the temperatures of a plant's own 
habitat are ordinarily the most favorable to it. It is likewise 
clear that the plant is subjected to a considerable range of tem- 
perature during the growing period. The point within this range 
at which the plant functions best is the optimum. This varies 
much, the optimum temperature for the seedling being lower than 
for the fruiting plant. It is never a mere point, but is a space of 
several degrees at least. In fact, for plants in their own habitats 
it is largely hypothetical. This is not true for the extremes of heat 
and cold which a plant can withstand and live. These are the 
maximum and minimum respectively, and are actual points beyond 



96 PLANT PHYSIOLOGY AND ECOLOGY 

which the plant dies. Naturally, maximum and minimum tem- 
peratures vary widely for different species, and to a small extent 
also for different individuals of the same species. Furthermore, 
a plant can withstand extremes of heat and cold much better in 
some stages than in others. It is least resistant in the active 
condition when the tissues are filled with water, and most re- 
sistant in the resting state typical of spores and seeds. Nearly all 
flowering plants are killed by exposure to temperatures below 
0° C. and above 45° C. Their seeds, however, may resist tem- 
peratures of —250° C. and 100° C. The resistance of seeds depends 
in a very large degree upon the absence of water. Seeds when 
quite dry have been found to survive when exposed for an hour 
to 110° C, while an exposure to water vapor at 70° C. killed 
them in one fourth the time. The maxima and minima of natural 
habitats during the growing period practically always fall between 
0° C. and 45° C. In hot springs the maximum for many algae 
and bacteria rises to 85° C. or higher. The bacteria are capable 
of withstanding much higher temperatures, and, in the spore 
condition, are able to resist temperatures of 120°-130° C. 

118. Freezing. The injury arising from exposures to low 
temperatures depends primarily upon the amount of water that 
the plant contains. For example, the watery leaves and stems 
are usually killed by exposure to 0° C, while the drier seeds and 
underground parts resist the long-continued action of tempera- 
tures from —30° to —40° C. Dry seeds, moreover, are capable 
of germination after exposure to —25° C, but soaked seeds lose 
this power under such conditions. The reason for this difference 
of behavior resides chiefly in the protoplasm, though it is impossible 
to go further than the statement that active protoplasm is more 
sensitive to cold than is the resting form. The small amount of 
cell-sap in resting tissues is also much more concentrated than 
that of living cells, and this of course increases the power of re- 
sistance. 

Frost, i.e., freezing temperatures, kills plants by withdrawing 
water from the cell-sap and forming ice crystals, usually upon 
the outside of the cells. When the freezing is extreme, the cells 
are ruptured by the formation of crystals within them. It is very 
probable that the chemical constitution of cell-sap and proto- 
plasm is changed in a manner harmful to the plant. Such a result 
seems to be clearly indicated by the behavior of many plants 



ADJUSTMENT TO TEMPERATURE 97 

which are seriously injured or killed by temperatures several degrees 
above the freezing-point. If allowed to become warm very gradually 
after freezing, many plants remain turgid in place of wilting, and 
thus survive. This result appears to be due to the gradual absorp- 
tion of the melting crystals by the cytoplasm, and there is in con- 
sequence no loss of turgidity. When the thawing takes place 
rapidly, the protoplast is unable to do this, and the ice crystals 
melt and fill the intercellular spaces. The cells become flaccid, 
and the plant wilts and dies. In the same habitat, plants differ 
much in their ability to resist frost, and this is also true of different 
parts of the same plant. Aerial parts naturally freeze first, but 
among these the newest and most active tissues, i.e., those con- 
taining the most water, succumb first. Flowers are most easily 
damaged, then the leaves, next the stems, and last of all the 
roots, which are more or less protected by the soil. Upon high 
mountains, where frosts may occur at almost any time during 
the summer, plants become unusually resistant, and able to with- 
stand repeated freezing and thawing. 

Experiment 32. Effects of freezing. Soak peas in water for twenty- 
four hours. Expose ten soaked and ten dry peas for twenty-four hours 
to temperatures below freezing. Place the two sets of peas in moist 
chambers, and compare their power to germinate. At the same time 
place several sunflowers of different ages out of doors, together with 
plants of a more woody species. Note the effect upon each. Transfer 
some plants quickly to a warm room, and others more gradually. Cut 
a section of a frozen leaf or stem, and compare with a section made of a 
part that has thawed out. 

119. The sum of temperatures. The greater number of species 
and of individuals pass through their entire life cycle without 
being exposed to maximum or minimum temperatures. Extremes 
of temperature have little significance for them. Their effect is 
confined to the plants that appear very early in the growing 
period, and those that linger toward the close. The activity and 
growth of any plant depend upon its receiving the requisite amount 
of heat during the growing period. Although temperature has no 
power to change the form and structure of plants, its influence 
upon size is very great, owing to its control over growth. The 
sum of the temperatures which act upon a plant is of the first 
importance in determining its general appearance. The effect 



98 



PLANT PHYSIOLOGY AND ECOLOGY 



may be produced either by temperatures that are more or less 
constantly too low, or by shortness of season, which is equally 
effective in reducing the total amount of heat available for the 
uses of the plant. As a rule, these two factors act in unison, 
producing marked reduction in size. Such reduction is character- 
istic of the vegetation of alpine regions, although the dwarf .habit 
of alpine plants is due chiefly to adjustment to water. The gen- 





** * 

A* 














**.*Bt , 


*£*J^*#£*^ 




■ ^ *r*f • ;l '^V : •V' 1 ' 


#K#-£5..f 


'.'■■• 




5pV 


- ^ -,• 1:- L : .J ,;'3 _ :~S*» 



Fig. 26. — Alpine mats. of Silene acaulis growing on the north side of a rock. 
The effect of different tempeiaiure sums is shown by the number of 
flowers. 



eral effect of low temperatures may often be seen in field crops 
during seasons in which the temperatures are largely below the 
normal. This effect may readily be demonstrated by growing 
seedlings of the same species in warm and cold compartments of a 
plant-house. The resistance of plants to low temperatures, espe- 
cially those of winter, depends upon the relation between the 
available water, which is reduced by freezing, and transpiration^ 
rather than upon the actual cold. 

120. Influence upon vegetation. Temperature has no effect 
upon the movement of plants, i.e., migration into new habitats, 
but it has a profound influence upon the establishment, or ecesis, 



ADJUSTMENT TO TEMPERATURE 99 

of the migrants. Seeds, fruits, etc., will move equally well in 
all directions, provided the proper agents are at hand for carry- 
ing them. Considering temperature alone, their ecesis is more cer- 
tain if they move to the east or west than if they move south- 
ward or northward. Generally, also, the chances of establish- 
ment are greater southward than they are northward. In the 
first case, migration to the east or west does not essentially change 
the relation of the plant to temperature. Migration southward 
means that the plant must accustom itself to higher temperatures 
as well as to a greater annual sum. While this is taking place, 
the plant may be at a disadvantage in competition with other 
plants already well established. Plants that migrate north- 
ward, in addition to a corresponding adjustment to lower tem- 
peratures and a lower sum, run an increasing risk of encountering 
a fatal minimum. This risk is greatly increased by the fact that 
southern plants require a longer period for their life cycle than 
northern ones, and in a northern habitat are often unable to reach 
maturity before the regular appearance of fatal frosts. So far 
as the controlling influence of temperature is concerned, plants 
spread readily to the east and west, less easily to the southward, 
and least easily to the north. The same fundamental rule applies 
to mountains, the increasing cold upward making ecesis more 
uncertain than it is downward. The grouping of species, which 
forms vegetation, is in accordance with this fact. In consequence, 
vegetation exhibits zones extending east and west upon conti- 
nents, and lengthwise along mountain ranges, which are due 
largely to temperature. The disturbing influence of other factors 
and the discussion of vegetation zones is considered under Zonation. 
121. Digestion. The complex materials, starch, cellulose, 
oils, and proteids, stored as reserve food in various parts of the 
plant, must be dissolved or otherwise chemically changed before 
they can be used. This process is digestion. It is carried on by 
the protoplasm or by certain peculiar products of it, called enzymes 
or ferments. In chlorophyllous plants, digestion takes place 
within the living cells, except in certain insectivorous forms where 
the secretion is poured out upon the surface, as in the pitcher- 
plant and Venus' fly-trap. Fungi, on the contrary, regularly 
carry on digestion outside of their own cells, either digesting the 
food before it is absorbed, as in many saprophytes, or making use 
of the enzyme to permit their entrance into the tissues of host- 
L/C. 



100 PLANT PHYSIOLOGY AND ECOLOGY 

plants. Every living cell of the plant possesses the power of 
digestion, but this is often localized in particular areas. It is 
inherent in chloroplasts and leucoplasts, and is present to a marked 
degree in the scutellum and aleurone layer of grains, in the outer 
cells of embryos surrounded by endosperm, in the digestive cells 
of insectivorous plants, etc. 

Enzymes possess the characteristic property of acting upon 
and changing an amount of stored food greatly in excess of their 
own bulk. The way in which they are formed by the protoplasm 
is unknown, but the production of a granular substance termed 
zymogen seems to be an essential step in the process. As a rule, 
each enzyme is able to digest but a single reserve material, or 
at most it is able to act upon only a few related materials. No 
enzyme known at present can act upon a carbohydrate and also 
upon oils or proteids. The enzymes which digest starch and 
related substances are termed diastases; that which attacks cellu- 
lose is called cytase. Invertase changes sucrose or cane-sugar into 
a simpler sugar. Lipase decomposes fats and oils, while pepsin 
and trypsin are the chief substances concerned in the transforma- 
tion of proteids. The chemical changes that take place in diges- 
tion are highly complex and still largely obscure. Solution is a 
characteristic step in the case of all solid food materials. Starch 
and cellulose are converted into sugars, which in their turn may 
be further changed before use. Starch is dissolved by the action 
of two enzymes, termed diastase of translocation, and diastase 
of secretion. The former occurs throughout the plant as well 
as in the germinating seed. The latter is largely confined to the 
seed, appearing in it only after germination has begun. Diastase 
of translocation attacks the starch grain uniformly, while diastase 
of secretion corrodes it in such fashion as to give it an irregular 
outline. 

122. Chemosynthesis of digested materials. Food material 
may be used in different ways by the plant, or even by the same 
cell. Sugar, for example, may be changed into a proteid and used 
to make additional protoplasm; it may be used to free energy 
by means of respiration, or to form cell wall, or it may be con- 
verted into various carbohydrates. While almost any soluble 
carbohydrate may be used directly to release the energy stored 
in it, it can be assimilated by the protoplasm, i.e., used in the 
construction of new living material, only through the chemical 



ADJUSTMENT TO TEMPERATURE 101 

addition of nitrogen to form proteids. Like nearly all chemical 
changes in the protoplasm, this process is little understood. 
Although it occurs regularly in the chloroplasts, it is not con- 
fined to them, nor indeed to green plants. It is not dependent 
upon light, and is termed chcmosi/nthcsis because the necessary 
energy is supplied by chemical action. The plant is unable to 
take nitrogen from the air, but secures it from the soil in the form 
of ammonia or of nitrates. The latter are combined with glu- 
cose or with maltose to form amides, diffusible nitrogenous com- 
pounds convertible into proteids. This action takes place in the 
presence of potassium, calcium, and magnesium, usually in the 
form of sulphates and phosphates. While all of these seem neces- 
sary, potassium alone appears to take part directly, though sul- 
phur and phosphorus appear ultimately in the protoplasm. The 
calcium is apparently for the purpose of neutralizing oxalic acid 
or other injurious compounds arising during the process. The 
probable reaction has been represented by the following: 

, , potassium . , potassium . , 

glucose + F nitrat-e = as P ara £ m + oxalate + wa ter + oxygen 

C 6 H 12 6 + 2*KX0 3 =C 4 H 8 N 2 03 + K2C2O4 +2H 2 0+ O^ 

123. Respiration. Light is the original source of energy for 
all chlorophyll plants, and indirectly for all hysterophytes, with 
the exception of a few nitrogenous bacteria. The energy of light 
is, however, available only at certain times. Consequently the 
plant has been obliged to find a way of storing it, so that it can 
be used at night as well as in parts of the plant deprived of light. 
This is brought about in the formation of complex food mate- 
rials which represent a certain amount of stored or potential 
energy. In such forms energy may be carried or translocated 
to various parts of the plant, and stored as starch, oil, or proteid 
to start the plantlet, so that it can again reach the light and obtain 
its energy directly. Translocation, storage, and digestion are all 
as much concerned with the distribution and use of energy 
as with that of food material that is to be assimilated. This 
is evident when it is called to mind that a sugar, oil, or pro- 
teid serves both as a supply of energy and as a constructive 
material. 

The liberation of energy stored in various compounds, which 
are originally soluble or rendered so by digestion, takes place 



102 PLANT PHYSIOLOGY AND ECOLOGY 

during the process of respiration. The latter is aerobic when it 
takes place in the presence of oxygen, and anaerobic when oxygen 
is absent. Aerobic respiration is characteristic of green plants, 
though these are to a certain extent anaerobic during germina- 
tion. Anaerobic respiration occurs chiefly in fungi. During 
respiration in flowering plants, oxygen is taken up from the air, 
and carbon dioxide is evolved, accompanied by the production 
of heat. The process is one of oxidation, in which the living sub- 
stance or the elaborated foods in it serve as fuel for the produc- 
tion of energy. It occurs in every living cell, but is most pro- 
nounced in regions of greatest activity, especially so in meristem. 
It is at a minimum in resting cells, and is practically absent in 
seeds and other propagative organs during the low temperatures 
of winter. Respiration is directly dependent upon temperature, 
but is little affected by light. The minimum temperature for the 
process is about — 15° C. Its activity increases with the tempera- 
ture, and appears to reach an optimum in the neighborhood of 
the maximum for flowering plants. 

124. Fermentation. Bacteria, yeasts, and molds obtain the 
necessary supply of energy by decomposing the greater part of 
the food upon which they grow into alcohol or various organic 
acids. This process, which is called fermentation, may be carried 
on by both aerobic and anaerobic plants, and is merely a kind of 
vigorous respiration. The power to carry on fermentation is 
small or absent in many fungi. It is especially characteristic 
of yeasts, and the consequent action is well illustrated by the 
common yeast. The latter decomposes sugar into alcohol and 
organic acids, with the evolution of carbon dioxide. Yeast fer- 
ments sugar and other carbohydrates normally in the presence of 
oxygen, but it may manifest this activity for some time without 
oxygen. Yeasts may act upon fats, but not upon proteids, while 
bacteria and molds produce fermentation in sugars, oils, or pro- 
teids. 

125. Germination. Seeds owe their ability to germinate under 
the proper conditions to the energy stored in the reserve food 
contained in them. This energy is released by the processes of 
digestion and respiration. The reserve food of seeds is usually 
in the form of starch, oil, or proteid, and in many seeds two of 
these occur together. It is either stored in the cotyledons or 
packed about them as endosperm. In both cases its digestion 



ADJUSTMENT TO TEMPERATURE 103 

is brought about by the cotyledons, which likewise absorb the 
digested material. When a large amount of food is stored in the 
cotyledons, the latter usually remain in the ground, or, if carried 
above it, they rarely become functional leaves. When the food 
is packed about the cotyledons, they act as the first leaves of 
the seedlings, except in the grasses. Here the single seed-leaf 
is transformed into a special organ of absorption, the scutellum. 
The digestion of stored material is carried on chiefly by the coty- 
ledons, though the cells of the endosperm also play a part when 
the latter is present. The conversion of starch into sugar is 
effected by diastase, largely secreted by the cotyledons. The 
latter often produce other enzymes as well, though these do 
not always seem necessary to the removal of proteids. The foods 
digested by the cotyledon are translocated to all parts of the 
embryo, and vigorous respiratory action is set up to secure the 
energy necessary for assimilation and growth. Such respiration 
is regularly aerobic, though under abnormal conditions it may for 
a time be anaerobic. Digestion continues until the food mate- 
rial has been removed from the endosperm or the cotyledons, by 
which time the seedling is provided with roots and leaves and 
is again able to obtain its food and energy directly from photo- 
synthesis. Respiration, on the other hand, is a continuous process. 
Once actively begun in the embryo, it continues throughout the 
life of the plant, disappearing only when the latter passes into 
the seed stage or other resting condition. 

Experiment 33. Digestion and respiration in seeds. Germinate seeds 
of the bean, sunflower, and Indian corn in a moist chamber or a ger- 
minator. As soon as the radicles appear, cut a median section of each 
seed to show the relation of the various parts. Note especially the 
condition of the reserve food as compared with that of the food in the 
dry seeds. Remove a few of the corn embryos, and place them upon 
moist potato-starch in a moist chamber. After some time, note the 
action upon the starch grains. 

Place a number of peas in a bottle, cover them with water, and close 
the bottle with a rubber stopper containing two holes. Insert a ther- 
mometer in one of the latter, lowering the bulb to the water. Bend a 
piece of glass tubing so that one end will fit in the second hole of the 
stopper without reaching the water below, and pass the other end into 
a stoppered bottle containing a 10% solution of lime-water or of barium 
hydrate. Explain the precipitate that is formed. Compare the tem- 
peratures of the germinating peas with those of the air outside. 



104 PLANT PHYSIOLOGY AND ECOLOGY 

126. Nutrition of hysterophytes. Plants that possess chlo- 
rophyll are holophytes, those that lack it, hysterophytes. From the 
standpoint of nutrition, plants that make their own food by 
photosynthesis are autotrophic, i.e., self -nourished, while colorless 
plants are heterotrophic, i.e., nourished by material traceable more 
or less directly to green plants. A few flowering plants are mixo- 
trophic, i.e., while they absorb most of their food in organic form, 
they are still able to make more or less sugar by photosynthesis. 
Some of these, such as the mistletoe, seem to show normal photo- 
synthesis, the parasite taking nothing but water and inorganic 
salts from the host-plant. Others, such as the dodder, are green 
only until they become attached to the host-plant, after which 
the chlorophyll disappears. Hysterophytes are usually grouped 
as parasites and saprophytes, although a large number of fungi 
may be parasitic or saprophytic, either by choice or by neces- 
sity. Many parasites are able to grow on a number of differ- 
ent host-plants, and certain of the saprophytic molds can nourish 
on almost every organic substratum. Other parasites, on the 
contrary, are confined to a single host, and occasional sapro- 
phytes are similarly limited in habit. 

A hysterophyte is nourished in practically the same way as 
the embryo which receives its food from the endosperm, or as 
the colorless tissues of a green plant, which are supplied with 
nutriment by the chloroplasts. The deep-seated cells of a wheat 
stem depend upon the leaves for food just as the rust upon it 
does, and the processes of digestion and of respiration are essen- 
tially the same in both. The saprophyte, though less directly, is 
similarly dependent upon the activity of green leaves for the sugar, 
starch, cellulose, oil, or proteid which it digests and absorbs. The 
respiration of many saprophytes is likewise similar to that of root- 
cells and of parasites, but in a large number, especially yeasts, 
molds, and bacteria, respiration implies fermentation. For this 
reason, saprophytes often grow readily in solutions containing 
organic acids, which are the products of fermentation. 

127. Kinds of parasites. The type of parasitism in which the 
presence of the parasite .benefits the host-plant in some measure 
is commonly distinguished as symbiosis or mutualism. Such a 
relation is found between certain fungi and the roots of many 
trees, the beech, oak, pine, spruce, etc., in Monotropa, and in the 
root-like stems of the coral-root, Corallorrhiza. The root with 



ADJUSTMENT TO TEMPERATURE 105 

its associated fungus is termed a mycorhiza. A similar relation 
exists between the roots of Leguminosce and the nitrogen bacteria 
which form tubercles upon them. Symbiosis occurs likewise 
between many of the simpler algae, such as Pleurococcus and Nostoc, 
and many cup-fungi and black fungi, constituting the forms termed 
lichens. In all of these cases the fungus is parasitic upon the 
host-plant, deriving from it all of its food, or in some cases the 
larger part of it. Its position in or about the host is of such a 
nature that it compensates the latter for the loss of food by some 
beneficial action upon it. The lichen surrounds the algal host 
with filaments in such fashion as to shield it from heat and drouth, 
though this relation was first established merely to withdraw 
carbohydrates from the algal cells. In Leguminosce the bacteria 
of the root tubercles make it possible for the plant to avail itself 
of the nitrogen of the air, while without the tubercles, as in the 
case of all other green plants, this substance can only be obtained 
from compounds in the soil. In return for the nitrogen fixation, 
the bacteria take their food supply from the host-plant. In the 
case of mycorhiza the fungus is probably a saprophyte originally, 
living upon humus in the soil. After finding its way into the 
root, it lives more or less parasitically, drawing all of its food, 
or much of it at least, from the host-plant, and in return aiding 
the latter in obtaining nitrogen compounds. 

Experiment 34. Nutrition of representative hysterophytes. Prepare 
cultures by half filling two small Petri dishes with a 2% sugar solution, 
and two with a thin flour paste. In a third pair place a layer of butter, 
and in a fourth a piece of moist cooked meat. Spread compressed 
yeast (Saccharomyces) over the various substrata in one series, and dust 
spores of blue mold (Penicillium) or black mold (Ascophora) over the 
material in the second series Place the cultures in a water-bath regu- 
lated for a constant temperature of 30° C, and note the growth from 
day to day. Compare the rate and amount of growth on the various 
substrata. Care must be taken to keep the cultures moist. 

Germinate sunflower seeds, and after the seedlings have appeared 
sow seeds of dodder (Cuscuta) in the same pot. Note the behavior of 
the dodder seedlings, and study the green, thread-like stem in cross- 
section. After it has become attached, cut a section through parasite 
and host at the point of union, and study the relation of the tissues of 
the two. 

Cut longisections of the root-like stems of Corallorrhiza, and note 
the position and relations of the fungal filaments. Cut cross-sections of 



106 PLANT PHYSIOLOGY AND ECOLOGY 

a lichen thallus, and note the arrangement of the host-cells. Carefully 
dissect the section in order to determine the connection between the 
fungal filaments and the individual aglal cells. 

128. Growth. The growth of any plant is made possible 
only by the growth of the individual cell, and, in multicellular 
plants especially, by the division of the latter. The absorption 
of water by a protoplast causes the stretching of the elastic cell 
wall, and the accompanying assimilation of food results in an 
increase in the amount of cytoplasm. The activity of the cyto- 
plasm brings about a deposition of cellulose particles in the stretched 
wall, and keeps it at its normal thickness. In a meristem cell 
that has stopped dividing and has begun to stretch in conse- 
quence of the absorption of water, the vacuoles of cell-sap in- 
crease in bulk more rapidly than the cytoplasm. The nucleus, 
moreover, shows a marked decrease in size, doubtless due to the 
withdrawal of material from it as it loses the power of active 
division. Thus, while the meristem cell contains a large nucleus 
surrounded by much cytoplasm filled with many small vacuoles, 
a single large vacuole is the most conspicuous feature of a paren- 
chyma cell. The cytoplasm, while it has increased somewhat 
in quantity, is now a thin layer closely applied to the inner sur- 
face of the wall, and the nucleus has become more or less incon- 
spicuous. In addition to the reinforcement of the cell wall by 
the placing of new cellulose particles among the original ones, 
the cytoplasm may add new layers of cellulose in those cells espe- 
cially destined for mechanical support. These thickening layers 
may be added almost uniformly, as in stone cells and fibers; at 
the angles, as in most thick-angled tissue; or in various forms, 
as in ringed, spiral, and reticulated vessels or tracheids. The 
first layers usually consist of cellulose, but in the later ones this 
is generally replaced by lignin in supportive tissues, and by cutin 
in protective ones, as in the cuticle of the epidermis. The growth 
of the cell becomes impossible after the wall is thickened or its 
substance changed, since mechanical stretching is no longer possible. 

129. Growth of tissues and organs. The continued growth of a 
mass, i.e., a tissue, is possible only when the individual cells in- 
crease in number as well as in size. Since increase in number 
is the regular consequence of the growth of the cell, the two always 
occur together. Increase in the size of a cell is limited not only 
by mechanical laws, but also, and especially, by the relation 



ADJUSTMENT TO TEMPERATURE 107 

between surface and volume. The volume grows so much more 
rapidly that the surface becomes unable to furnish adequate 
food material, and division becomes necessary as a temporary 
remedy for this condition. In all cells, division is brought about 
by a preceding indirect division of the nucleus, i.e., mitosis, though 
in a number of cases the nucleus divides directly without affecting 
the cell. All the cells of the embryo, for a time at least, are ca- 
pable of active division, i.e., they constitute meristem or dividing 
tissue. Even in the seedling, the greater number of the cells 
have lost this power, which is confined to a few special regions 
during the further development of the plant. Practically all vascu- 
lar plants maintain meristem at root-tips and stem-tips through- 
out the life period. With the exception of monocotyledons, 
woody spermatophytes and many herbaceous ones retain in the 
stem a special meristematic layer, the cambium. The ordinary 
shrubs and trees possess meristem in their buds and in the layer 
which produces the protective cork. In roots the pericycle is 
persistently meristematic, and the inner layers of the cortical 
cylinder sometimes possess the same property. The parenchyma 
of both stem and root for a long time possesses the power to pro- 
duce meristem, and it regularly serves this function in producing 
the cambium which connects the bundles of the stem. In flower- 
ing plants the meristem of all growing tips proceeds from one 
or more groups of apical cells, while in the mosses, liverworts, 
and ferns it is derived from the division of a single apical cell. 
In all plants higher than the simple algae, the stimulus of fer- 
tilization produces growth. In the carpophytes the action is 
exerted both upon the egg-cell and one or more of the adjacent 
cells; in mosses and ferns the resulting growth is usually con- 
fined to the egg-cell. In the spermatophytes, fertilization initi- 
ates the development of the endosperm as well as that of the 
embryo, and likewise often produces a striking growth of calyx, 
receptacle, or other portion of the fruit axis. Both propagation 
and reproduction are consequently to be classed as phenomena 
of growth, and to be considered in connection with it. 

Experiment 35. Regions of growth. Sow fern spores on clean, moist 
sand, and after they have germinated, observe them from time to time, 
noting the behavior of the apical cell. Germinate seeds of the horse- 
bean (Vicia faba), and after the seedlings appear, cut off the root and 
stem tips, as well as some sections of the upper part of the radicle. 



108 PLANT PHYSIOLOGY AND ECOLOGY 

Embed and cut on the microtome. Study the origin and structure of 
the meristematic regions Cut cross-sections of a woody stem for the 
study of cambium, and the cork meristem or phellogen. Cut cross- 
sections of a flower bud and note the meristem. 

130. Conditions that influence growth. In most one-celled 
plants, growth takes place equally in all directions. In fila- 
mentous and massive forms, growth is greatest in one direction, 
and division is correspondingly modified. The cell and the plant 
become more or less drawn out, as though influenced by two 
poles. Although this phenomenon, which is called polarity, is 
all but universal among plants, its cause is obscure. The polarity 
so characteristic of stemmed plants is largely a matter of the 
control exerted by gravity, but in the case of thallophytes, e.g., 
liverworts, lichens, and algae, other factors, light, water, etc., enter 
in. Gravity is the most important of all forces in determining 
the direction of growth. Broadly speaking, the main axis of 
vascular plants is in line with the pull exerted by gravity, while 
the secondary axes, branches, leaves, etc., are more or less at 
right angles to it. The growth of the root is with the pull due 
to gravity, that of the shoot against it. The relation of growth 
to gravity will be further discussed under geotropism. 

131. The amount and rate of growth are determined by the 
physical factors of the habitat. Growth is directly affected by 
the condition of the plant, especially its turgidity and nutrition, 
but these are under the control of the physical factors. The sup- 
ply of oxygen is an important factor, though its effect is observ- 
able only when the usually adequate supply is greatly reduced, 
as in the case of many water plants and humus plants, particu- 
larly certain saprophytes. Water is, for many reasons, of the first 
importance in growth. It is necessary for the mechanical stretch- 
ing of the cellulose wall, which is a prerequisite for the growth 
of all tissues. It is not only necessary for the formation of car- 
bohydrates, but also for the absorption of nitrogen compounds 
and other necessary salts. It serves as a vehicle for the trans- 
location of elaborated foods, and is indispensable in maintaining 
the turgidity of the plant. Temperature is equally important in 
growth. Apart from the indirect effect which it exerts through 
its influence upon absorption, photosynthesis, etc., it controls 
growth directly through digestion, assimilation, and respiration. 
Indeed, heat and water may be termed the two requisites for 



ADJUSTMENT TO TEMPERATURE 



109 



growth. Light apparently exerts only an indirect effect through 
the dependence of growth upon the food supply furnished by 
photosynthesis. Strong sunlight has been supposed to exert a 
retarding influence upon growth, but this is very doubtful. In 
diffuse light and darkness, plants grow taller than in sunlight, 
but this result seems to be due to other causes, and only indirectly 
to light. Humidity acts indirectly but powerfully upon growing 
plants by controlling transpiration, and through it affecting absorp- 
tion. Soil influences growth by its control of water content, air 
content, and nutrient salts. Except for its occasional mechanical 
action upon the growth of woody plants, wind, like the remaining 
physical factors, affects growth only through other factors. 

132. Regions of greatest growth in various organs. The 
growth in length of roots, stems, and leaves is normally due to 




Fig. 27. — Seedling of the horse-bean, Vicia faba, showing the amount and 
location of the growth in A after 6 hours (B) and after 24 hours (C). 
(After Pfeffer.) 

the apical meristem. In some cases layers of cells at one end 
of the internodes of the stem or near the bases of the leaves re- 
tain the power of growing, and thus produce intercalary growth. 
In all regions of growth, and especially so in apical ones, there 
are two more or less distinct zones. In one the cells divide actively, 



110 PLANT PHYSIOLOGY AND ECOLOGY 

but grow little; in the other division is greatly diminished or 
altogether lacking, and the stretching of the cells pronounced. 
The former consists almost wholly of meristem and may be called 
the zone of division, while the latter consists of cells more or less 
modified into parenchyma, and may be termed the zone of elonga- 
tion. The zone of division is regularly much shorter than that 
of elongation, the ratio often being greater than 1 : 3. The zone 
of division occupies the tip of stem or root, and the zone of elon- 
gation extends back from it for a distance several times greater. 
Elongation is greatest just back of the meristem, and it decreases 
gradually toward the permanent tissue of the organ. In the 
intercalary growth of an onion leaf, elongation is greatest at the 
apex of the sheath, and it decreases less rapidly upward into the 
blade and more rapidly downward. The length of the growing 
region varies for different plants. In roots it may vary from 
one to several centimeters. The growing region of the stem is 
normally much longer, ranging from 5-40 centimeters and in 
rare cases even more. It may consist of one to several inter- 
nodes, and in certain water plants growth may extend over as 
many as fifty internodes or rarely over the entire stem. 

Experiment 36. Influence of temperature, water, and light upon regions 
of growth. Germinate seeds of Vicia faba in a moist chamber, and as 
soon as the roots are a centimeter long, mark several carefully with an 
inked thread at intervals of a millimeter. Put one or two in a warm place 
and as many in a cool spot Determine the region of greatest growth, 
and note the differences due to temperature. 

Select four sunflower seedlings that show three internodes, taking 
pains to choose plants as nearly alike as possible. Mark the stem of 
each at 5 mm. intervals, and mark one of the youngest leaves of each 
transversely at intervals of 2 mm. Place one plant in a warm sunny 
place and keep it well watered, marking the other leaf of the upper 
pair longitudinally at intervals of 2 mm Place the second plant along 
side the first, but do not water it. Put the third plant in darkness, 
and the fourth one in a cool spot. Follow the growth in each from day 
to day. After several days or a week determine the total growth of 
stem and leaf under each condition, as well as the region of greatest 
growth for stem and leaf. 

133. Rhythm of growth. The rate of growth is by no means 
uniform throughout the day or the year. It exhibits a certain 
rhythm or periodicity, in accordance with which the growth flue- 



ADJUSTMENT TO TEMPERATURE HI 

tuates greatly at different times, or ceases at the end of a certain 
period. The division of the year into a growing period and a 
resting period is the most striking evidence of the law of growth. 
Variations in growth probably arose originally from the fluctua- 
tion of the controlling factors, heat and water. They have become 
so firmly impressed upon the plants through the action of simi- 
lar fluctuations upon countless generations that they are more 
or less firmly fixed as habits which persist under changed condi- 
tions. In consequence, rhythm appears to depend as much or 
even more upon the habitual development of the plant as upon 
the physical factors of its habitat. 

The daily growth of a plant shows a maximum and a mini- 
mum, the latter sometimes falling to zero. The maximum usually 
occurs late at night, apparently after translocation becomes less 
active, and the minimum falls near noon, not far from the period 
of greatest photosynthetic activity. Between the two the rate 
of growth falls and rises more or less gradually, though abrupt 
changes often occur. During the growing period growth begins 
slowly, but after a certain period, the length of which varies for 
different plants, it rises rapidly and uniformly, as a rule, to the 
maximum. The latter rate is maintained for a short time only, 
after which it first falls rapidly and irregularly to a point near 
the minimum. Growth is then maintained at a very low, irregu- 
lar rate for some time and finally stops altogether. Cessation of 
growth may occur but once in the lifetime of an individual, as 
in the case of annual plants and most thallophytes, or it may 
occur at fixed intervals, as in perennial herbs and woody plants. 
In the latter the rhythm shown in a single period of growth and 
of rest lasts throughout the entire year, although unusual condi- 
tions may cause an interruption of growth at any time. Changes 
or conditions of the plant which are connected with the resting 
period become fixed habits, owing to their constant recurrence. 
This is equally true of the adult plant and of the embryo in the 
seed. Woody plants which shed their leaves at the beginning 
of an annual period of cold or drouth often retain this habit after 
the cause is removed. Seeds which have been regularly exposed 
to winter conditions have acquired the habit of remaining dor- 
mant during this period. When brought under the usual condi- 
tions they may refuse for a long time to germinate unless they 
are subjected to cold naturally or artificially. The daily rhythm 



112 PLANT PHYSIOLOGY AND ECOLOGY 

of growth or movement also becomes fixed, and is often exhibited 
by plants in the absence of the usual causes. 

134. The age of a plant, i.e., the total period of growth, depends 
very largely upon its size and complexity. A unicellular plant, 
such as a bacterium or an alga, may pass through its life cycle 
in a few minutes, for, while it does not die, its individuality is 
lost in consequence of division into two new cells. At the other 
extreme are found the trees, many of which live for centuries, and 
a few for more than a thousand years. Practically all vascular 
plants require at least one season to complete their development, 
and the majority of them live for many years. The death of plants 
which have but a single period of growth is ordinarily due to 
unfavorable physical factors, or to the fact that all the tissues 
of the parent plant have taken the permanent form, leaving no 
meristem to initiate growth during the following season. Woody 
plants might well seem to be immortal, except perchance for 
accidents, but even in them the great accumulation of material 
sets a mechanical limit to the size that can be attained. The 
approach of this limit is furthermore hastened by the inevitable 
decay of the dead tissues of the trunk, resulting in the fall of the 
tree while growth is still possible. 

135. Reproduction. The earliest division of labor in plants 
produces a nutritive or vegetative part and a reproductive one. 
The two are absolutely interdependent: they are connected by 
growth, which is the result of nutrition and the cause of reproduc- 
tion, or at least the means by which it is brought about. The 
simplest case of reproduction is that shown in fission, where the 
production of two new plants from the parent cell is the direct 
outcome of growth. Multiplication must have been originally at 
least merely a consequence of this process, by which a plant was 
enabled to continue growth by becoming two. Since growth is 
characteristic of all plants, reproduction in direct consequence of 
growth is found in practically all groups of plants. This process 
is ordinarily termed asexual reproduction or propagation, and 
the cells or parts by which it is carried on are propagules. The 
formation of propagules can take place only during the period of 
growth, and these serve, as a rule, for multiplication under favor- 
able conditions. . A large number of propagules, however, pass 
into a resting condition by the formation of a protective covering, 
and thus serve to carry the plant or a portion of it through drouth 



ADJUSTMENT TO TEMPERATURE 113 

or cold. The last, however, is the usual task of sexual reproduction 
or generation. 

The fusion of two sexual plasms or gametes produces a sporo- 
phore. In the phycophytes the latter is one-celled and is usu- 
ally spoken of as a resting spore. In all cases the sporophore 
or some part of it is well protected, to avoid drying out during 
unfavorable conditions, usually those of winter. It is also stored 
with food, or contains chloroplasts to enable the protoplasm or 
plantlet to burst the covering and to obtain a good start when 
favorable factors return. In some algae and fungi the entire 
sporophore assumes this role, but in the majority of these, and 
always among the mossworts and ferns, this task is assigned to 
the spores produced by it. Among flowering plants it is the seed 
that serves this purpose. In the following brief discussion of the 
methods of reproduction, it is not feasible to consider all of these, 
and only the more important are indicated. 

136. Propagation. Fission is the almost universal method of 
propagation among the unicellular plants. Budding is merely a 
kind of incomplete fission, and the internal division of the proto- 
plasm in forming macrozoogonids is practically fission within 
the cell wall. After plants became multicellular, however, fission 
merely increased the number of cells in the plant, except in those 
cases where the cells fall apart to form conidia. Filamentous 
forms consequently came to break their threads into pieces or 
hormogones by the modification or death of a cell. Among the 
multicellular phycophytes the propagules are usually asexual 
zoospores, while among the carpophytic fungi the latter have 
become colorless and aerial, constituting conidia. In both cases 
they are the direct consequences of growth, and are designed for 
immediate germination under favorable conditions, as well as to 
bring about distribution. Conidia, however, have acquired more 
or less resistance, since a long time may often elapse before they 
reach proper conditions for germination. 

Among the liverworts and mosses the propagules are usually 
special bits of the plant body or gametophore, which are called 
gemmae. These are usually formed upon the thallus or the stem, 
though they are also developed from other parts. When detached 
they ordinarily grow at once into new plants. In a few cases 
specialized leaves may serve the same purpose. The fernworts 
and ferns have placed their stems underground in most cases, 



114 PLANT PHYSIOLOGY AND ECOLOGY 

and propagation is largely carried on by the latter. In conse- 
quence, other kinds of propagules are rare. Buds, bulbils, or 
bud-like bodies serve for propagation in some ferns. 

137. Propagules of flowering plants. In the spermatophytes, 
root, stem, leaf, and flower are all capable of producing meris- 
tematic tissues that may serve as the essential part of a propagule. 
Probably no plant has the power to develop buds upon all of these 
organs, and in most cases this ability is restricted to one or two 
parts. It is a common characteristic of the stem and is often found 
in roots. It occurs rarely in leaves, at least under natural con- 
ditions, and is altogether exceptional in flowers. Root buds, 
i.e., the use of roots as propagules, are very common in woody 
plants, both trees and shrubs, such as the aspen, cottonwood, 
lilac, rose, raspberry, etc., and it occurs also in herbaceous plants 
e.g., milkweeds, dock, toadflax, and certain composites. The 
development of plants from root buds often takes place under nor- 
mal conditions, but it is a regular occurrence when the trunk 
has been cut down. 

Buds occur on foliage leaves, especially those that are thick 
or fleshy. They spring usually from the margins, but are also 
formed upon the surface. Such leaves are regularly used for 
artificial propagation by florists and gardeners. In nature leaf 
buds occur regularly on only a few plants, though they have been 
occasionally observed upon a number of herbs, chiefly among the 
mustard and lily families. Although some are able to form 
plantlets upon the leaf while it is still in position, it is evident 
that propagation by this method would rarely prove successful 
in nature, except perhaps in wet habitats. The production of buds 
from floral leaves occurs in but a few genera, and such buds are 
incapable of propagation without the artificial removal of the 
carpels. Flowering branches are often replaced by propagative 
buds in many grasses, in the onion, saxifrage, etc. 

138. Stems as propagules. Bud-bearing stems are variously 
modified to constitute propagules, of which they are by far the 
most important source. According to the form which the stem 
takes, such propagules are termed stolons, offsets or runners, 
rhizomes, corms, bulbs, and bulbils. The stolon is a descending 
or trailing leafy branch which forms roots and a shoot where it 
touches the ground; it is found in many bushes, currants, goose- 
berries, etc . The runner is a prostrate or decumbent slender leafless 



ADJUSTMENT TO TEMPERATURE 115 

stem, which forms roots and leaves at its outer end, either before 
or after the latter touches the ground. Offsets are short runners, 
but there is no real difference between the two. Runners are found 
in the strawberry, certain species of erigeron, saxifrage, etc. 
Rhizomes or rootstocks are the underground stems or branches 
so characteristic of perennial herbs, e.g., most grasses, mints, 
iris, Solomon's seal, etc. They persist from year to year, forming 
new buds and carrying the successive generations further and 
further from the original home. A tuber is a greatly thickened 
rootstock, or a thickened portion of one, such as is found in the 
potato. The corm is really a short tuber, often broader than 
long, and usually upright in position. Roots are produced from 
the lower surface and buds from the upper, though they may 
spring from the sides also. Corms are found in the crocus, jack- 
in-the-pulpit, etc. Bulbs resemble corms in shape, but they are 
not solid. They are greatly shortened subterranean stems, made 
up largely of thickened scale-like leaves. Bulbs are the charac- 
teristic propagules of the lilies and their relatives. Bulbs and 
corms sometimes form underground offsets which produce new 
bulbs at the end. Bulbils or bulblets are small aerial bulbs, 
produced in the axils of leaves, as in the lily, or in flower clusters 
as in the onion. 

Experiment 37. Propagules. Note the development of the perennial 
herbs and the shrubs in the spring. Determine the method of propaga- 
tion in as many as possible. Prepare a list for grassland and forest 
of the plants thus studied. Arrange the species according to the type 
of propagule, and note the distribution and importance of the various 
types. 

139. Sexual reproduction. In its simplest form sexual re- 
production is merely the fusion of tw r o protoplasts or gametes. 
There is no differentiation of the gametophore, and fertilization 
has no effect apart from the two cells concerned. In the beginning, 
sexuality seems to be little more than a device by which a double 
quantity of protoplasm is secured for the resting spore. In the 
carpophytes, and especially in the bryophytes, the gametophore 
is considerably differentiated. Fertilization produces a sporo- 
phore of increasing complexity, in which spore production, though 
still the principal function, is not the only one. Step by step 
the sporophore assumes the functions of the gametophore, until 



116 PLANT PHYSIOLOGY AND ECOLOGY 

the latter is left only its characteristic task, the production of 
gametes. During this time a division of labor occurs by which 
each gamete is developed upon a special gametophore. With 
the appearance of the flowering plants, the primitive relation 
in which the sporophore is dependent upon the gametophore 
is reversed. The macrospore is retained upon the sporophore, 
and the male gamete is no longer able to reach the egg-cell by 
swimming through the water to it. The sporangia in which micro- 
spores and macrospores are produced are further protected and 
nourished by being enclosed within the sporophylls that bear 
them, giving rise respectively to stamens and pistils. A further 
step in the increase of parental care leads to the loss of the power 
to produce sporangia by some of the sporophylls, which thereby 
become sepals. The need of insuring the transfer of pollen grains 
from stamens to pistils has apparently produced a further division 
of labor. The innermost sepals have become brightly colored 
in the majority of cases, and as petals serve as organs of attraction 
for insects, as well as for other animals that bring about pollina- 
tion. The flower is hence to be explained as a reproductive 
device, by which the sporophore secures better protection and 
nutrition for its spores and gametes, and insures the fusion of 
the latter in spite of changed conditions and the loss of motility 
in the male gamete. 

The immediate task of the flower is to bring about pollination 
and consequent fertilization, by means of which seeds and fruits 
are produced. To secure the proper discharge of these functions, 
the flower has undergone innumerable modifications. All of 
these may be grouped with respect to: (1) the production of 
pollen, (2) its protection, (3) the disposition of stamens and petals, 
(4) source and destination of pollen, (5) relation to the agent 
concerned in dispersal. 

140. Production of pollen. Pollen grains are commonly 
exposed to the double risk of injury by weather and of loss in 
transit, particularly in transfer by winds. Furthermore, they 
often serve as food for the insect agents of pollination, and a large 
number of grains are thus sacrificed in order that a few may be 
carried. As a general rule, the amount of pollen produced 
increases with the danger of loss. There are few if any definite 
modifications for this purpose, doubtless because it is most 
easily accomplished by increasing the number of stamens in each 



ADJUSTMENT TO TEMPERATURE 



117 



flower, or of stamen-bearing flowers. A large production of 
pollen is secured in many open flowers, such as the buttercup, 
anemone, strawberry, cinqfoil, etc., by a large number of stamens, 
and doubtless compensates for the great loss arising from the ease 
with which many insects may reach the pollen and eat it. Among 
wind-pollinated plants the greatest loss occurs in the dioecious 
trees, such as the cottonwood and ash, and the monoecious conifers^ 
e.g., pines, spruces, and firs, in which the pistillate cones are 





Fig. 28. — Extremes of pollen production. 1, an orchid, Orchis sp.; p, pol- 
len mass in anther cell; r, retinaculum; s, stigma; 2, a baobab flower 
(Adansonia) with a column of stamens. (1 after Le Maout and De- 
caisne, 2 after Baillon.) 



usually above the staminate ones. The need of compensation 
in such cases is very great, and the amount of pollen necessary is 
enormous. Indeed, in many coniferous forests, nothing is found 
so universally scattered as pollen grains that have miscarried. 
In many trees the great loss of pollen is offset by the development 
of a large number of flowers, especially of imperfect ones in which 
the material ordinarily available for sepals, petals, and pistils is 
used for stamens. This is the case in polygamous flowers, such 
as those of the maple. Not all wind-pollinated flow r ers produce 
a large amount of pollen. In the grasses and sedges, for example, 
the number of stamens is usually 1-3. This is probably to be 
explained by their habit. They are low-growing and usually 
densely associated, in consequence of which the pollen is carried 
through the whole group of individuals before it is blown away. 
As would be expected, the number of stamens and hence the 



118 PLANT PHYSIOLOGY AND ECOLOGY 

amount of pollen produced decreases as the method of pollination 
becomes more and more perfected. Moreover, accessory staminate 
flowers disappear, and the number of flowers is often greatly 
reduced also. In practically all zygomorphic flowers, i.e., those 
that apparently owe their irregular form to insect-pollination, 
such as the mints, snapdragon, orchids, etc., the number of 
stamens is regularly reduced to four or two. In certain orchids 
the number of pollen masses, or pollinia, is reduced to two or one, 
so certain has pollination become. 

Experiment 38. Amount of pollen. Make a comparison of various 
species with respect to the number of flowers, and the number of stamens 
in each flower Estimate the number of pollen grains in an anther 
and determine the total pollen production of a plant of each type. 

141. Protection of pollen. Flowers protect their pollen against 
injury from rain or dew by means of the most diverse modifica- 
tions. In many instances the protection afforded is secondary, 
the structure or modification having been developed chiefly for 
other reasons. The most striking devices, especially those involv- 
ing a movement of the plant or its parts, serve primarily for pro- 
tection. A number of plants, particularly wind-pollinated ones, 
seem to have no protection against injurious moisture. In many 
of these it will probably be found that the protection, though 
obscure, is effective. The period of flowering and the time of 
flowering, when more thoroughly investigated and understood, 
will suffice to explain why some flowers seem unprotected. It 
is probable that plants in which the pollen is not protected from 
harm in some way do not occur, since the existence of a species 
is proof of such protection. In some plants indifference to the 
effect of rain or dew apparently constitutes an ample protection. 

The damage that results from wetting is not the same for 
every plant. In some, moisture causes premature germination; 
in the majority it interferes seriously with the transfer of the 
pollen. In all these cases the protection afforded the pollen serves 
also for the nectar. In some instances it is possible that the 
device has really been developed for the latter purpose. 

Pollen must be protected against dryness as well as against 
moisture. This is usually accomplished so effectually by the 
structure of the grain itself that other modifications for this purpose 
are obscure, if not altogether lacking. In the case of desert plants 



ADJUSTMENT TO TEMPERATURE 



119 



it Is possible thai the period or time of flowering, the structure or 

position of the flower, may result in a certain degree of protection. 
The stigma, while in a receptive. condition, must be guarded against 
excessive dryness, and also perhaps against rain. This is usually 
brought about by the same devices that insure the protection 
of the pollen. Finally, the pollen must be guarded against those 
insects which would devour it without effecting its transfer. This 
result is usually obtained as a secondary consequence of modifica- 
tions for insuring pollination. 

142. Structural protection. The devices which serve to protect 
pollen are of three sorts: (1) morphological, (2) mechanical, (3) 




Fig. 29. — Structural protection of pollen in the bearberry, Arctostaphylus 
uva-ursi, by means of the inverted flask-shaped corolla. 



seasonal. The first and third, as a rule, accomplish protection 
incidentally. Structures of the second class probably owe their 
very existence to the necessity for protection of the pollen. Mor- 
phological contrivances are purely structural or positional. To 
the first class belong all flow T ers in which protection results from 
the structure or shape of the flower, of the flower parts, or of the 
flower cluster. Protection of this sort may arise from the structure 



120 PLANT PHYSIOLOGY AND ECOLOGY 

or sculpturing of the pollen grain itself, or from the form of stamen ? 
pistil, corolla, calyx, bract, or inflorescence. The thick wall of 
the pollen grain is a very considerable protection, often increased 
by oil or viscin, as in Onagra, Circcea, etc. Protection is also 
brought about by the position of the anther, or through the location 
of the pore by which the latter opens. In conifers the swelling 
of the staminate scales when moist closes the way to the pollen 
grains. 7ns covers the stamen with a broad petaloid stigma. 
Instances of protection by means of the shape of the corolla occur 
abundantly among flowers with united petals, e.g., Androsace 
Gentiana, Lithospermum, etc., and sometimes among those with 
separate petals, such as Aconitum, Bicuculla, and Delphinium. 
Certain cleistogamous, i.e., closed, flowers, also belong here. More 
rarely the calyx serves the same purpose, as in some species of 
Clematis. In Ariscema and other Aracece, protection is brought 
about by the form of the spathe. In certain catkin-bearers, 
Populus especially, some shelter is afforded by the large bracts. 
Protection as a result of the position of the flower or inflorescence 
occurs in a large number of species in which the flowers are turned 
toward the earth, and in flowers with tubular corollas which de- 
viate even slightly from an upright position. To this class belong 
Erythronium, Pentstemon, Teucrium, species of Gilia, etc. In 
Tilia the flower clusters are placed in such a position that they 
are sheltered by the leaves, a device which also occurs in some 
species of Impatiens. 

143. Protection by movement. Mechanical devices comprise 
movements of the flower or its parts, or of the flower cluster. The 
movement is one of closing or of drooping. In most cases pro- 
tection takes place by the closing of the corolla, more rarely by 
the closing of other parts of the flower. This is especially well 
shown in day-bloomers and night-bloomers, particularly those 
that are ephemeral, i.e., that wilt shortly after opening. Day- 
blooming and night-blooming serve to protect the pollen during 
the time when it is not being removed and may be injured, although 
this habit was probably first acquired with relation to insect 
visitors. The sepals sometimes close for protection in apetalous 
forms. The swelling of scales in pine cones and the closing of 
pores in some anthers should be mentioned here, though they are 
not due to a definite movement. The protection which is obtained 
in some flowers by a pendulous or ascending position is secured in 



ADJUSTMENT TO TEMPERATURE 121 

many erect ones by the bending or drooping of the flower-stalk. 
This is notably the case in Campanula rotundijolia and to a less 
extent in C. aparinoides, in which the bud is erect but the flower 
is drooping. In Oxalis, Anemone, etc., the bending takes place 
more quickly and lasts overnight only, or, more rarely, throughout 
cloudy or rainy days. In some cases the entire flower cluster 





a 

Fig. 30. — Protection of pollen in the California poppy, Eschscholtzia cali- 
fornica, by the rolling of the petals in -wet weather (b). (After Kerner.) 

droops, as in certain geraniums, umbellifers, and composites. In 
some radiate and ligulate flowers of the last family, protection is 
afforded by the upward or inward movement of the ligules. In 
radiate flowers, such as the asters, the protection of the disk 
florets by the rays is only partial, but in species of the Liguliflorce 
each floret is protected by its own ligule and by the longer ligules 
of the florets nearer the margin. 

The movements of the flower and its parts are usually referred 
to as anihotropism. The mechanism by which the movement is 
produced is not well understood in most cases. The opening and 
closing of the flower, as well as the bending of the rays in com- 
posite heads, is now definitely known to be caused by variations 
in temperature, and not in light, as is commonly supposed. The 
precise nature of the response to heat is not known, but it seems 
to be a direct reaction of the protoplasm. The bending of flower- 
stalks may be due to more rapid growth upon one side than the 
other, or to the attraction exerted by gravity. 

144. Seasonal protection. The seasonal protection exhibited 
by many flowers is concerned with the time or period of flowering. 
This is shown in plants such as Ipomoea, Taraxacum, etc., which open 
their flowers in bright sunshine and close them upon the approach 
of rain or at nightfall, thus effectively sheltering the pollen. 



122 PLANT PHYSIOLOGY AND ECOLOGY 

Ephemeral flowers, e.g., Tradescantia, Portulaca, Onagra, etc., 
open only under favorable conditions, and wilt after a few hours, 
thus reducing the chance of injury to a minimum. The blooming 
of flowers at particular times during the growing period doubtless 
has some connection with the presence or absence of definite pro- 
tective devices. Many species have formed the habit of flowering 
at the season in which there is least danger from rains. Seasonal 
protection sometimes occurs along with morphological or mechan- 
ical devices, producing a double protection. 

Experiment 39. Protection of pollen. Make careful field observa- 
tions upon the spring flowers, and note whether the pollen is sheltered 
or not. Study and sketch some of the ways in which protection is 
brought about, and arrange the species in groups according to the 
method employed. 

145. Disposition of the stamens and pistils. Since the pollen 
must be transferred from the anthers to the stigma in some way 
in order to insure fertilization, the relative position and develop- 
ment of the stamens and pistils become matters of the greatest 
importance. They not only affect the method of transfer, but 
they also determine the kind of fertilization that results. The 
arrangement by which stamens and pistils occur in different 
flowers is termed diclinism, and plants which possess staminate 
and pistillate flowers are said to be diclinic. Diclinic species 
are monoecious when the staminate and pistillate flowers occur 
upon the same plant, and dioecious when the staminate flowers 
grow upon one plant and the pistillate ones upon another. Plants 
with stamens and pistils in the same flower are said to be mono- 
clinic. The presence of monoclinic and diclinic flowers in the 
same species is called polygamy. In monoclinic or perfect flowers 
the rule is for stamens and stigmas to mature at different times, 
in order to increase the chance of cross-pollination. This condition 
is termed dichogamy. Dichogamous flowers are protandrous when 
the anthers shed their pollen before the stigma becomes receptive, 
and protogynous when the stigma matures first. The stigma has 
been termed short-lived when it is receptive for a short time, 
and long-lived when it remains receptive, i.e., capable of caus- 
ing pollen to germinate, for several days or more. When the 
anthers and stigma mature at the same time, the flower is termed 
homogamous. 



ADJUSTMENT TO TEMPERATURE 123 

Flowers usually open before or upon the maturity of anthers 
or of stigma, but sometimes they remain completely or partly 
prosed and are called cleistogamous. Homogamous flowers, which 
open, may have the anthers and stigma so placed that self-pollina- 
tion is impossible, or these parts may be contiguous in such fashion 




Fig. 31. — Dichogamy in the fireweed, Chamcenerium angustifolium, in 
which the anthers mature before the stigma. (After Kerner.) 

that self-pollination may occur. Flowers that may be self-polli- 
nated sometimes have the stamens, or the stamens and the styles, 
in two or three sets of different lengths, in a measure decreasing 
the disadvantage of self-pollination. 

Experiment 40. Grouping of stamens and pistils. Note the relations 
of stamens and pistils in various flowers of the spring flora. Make 



124 PLANT PHYSIOLOGY AND ECOLOGY 

sketches of several flowers representing different types, showing the 
flower in longitudinal section. 

146. Source and destination of pollen. When a stigma receives 
its pollen from the anthers in the same flower, the flower is self- 
pollinated. If the pollen comes from a different flower, the flower 
is cross-pollinated. Self-pollination is also called autogamy, and 
cross-pollination, allogamy. The latter is the rule among plants, 
though autogamy is a frequent occurrence. A great number of 
plants that are modified for allogamy and are regularly pollinated 
in this way are so arranged that they may be self-pollinated in 
case cross-pollination does not occur. Darwin was the first to 
show conclusively that cross-pollination tends to produce stronger 
and more vigorous plants, thereby furnishing an adequate ex- 
planation of the preference which plants have for this method. 
The numerous modifications of diclinism and dichogamy seem to 
be for the purpose of preventing self-pollination, while the in- 
numerable devices for dispersing pollen, attracting insects, etc., 
are to insure cross-pollination. All diclinic plants and many 
dichogamous ones can be pollinated in this way alone, while allo- 
autogamous species, i.e., those capable of pollination in either 
way, are self-pollinated only after the failure of cross-pollination. 
Many flowers belonging to different varieties, or more rarely to 
different species, may cross-pollinate each other. Although the 
crossing of related forms has been an invaluable method in plant- 
breeding, it does not seem to be a frequent process in nature, 
apparently being limited to a few genera, such as the willows, 
verbenas, etc. Cross-pollination of this sort is termed hybridiza- 
tion. 

Cross-pollination between two flowers of the same plant is 
called geitonogamy, i.e., pollination by a neighboring flower; between 
flowers of different plants it is xenogamy, i.e., pollination by a dis- 
tant flower. Either may occur in species with perfect or monoe- 
cious flowers, but xenogamy alone is possible in dioecious plants. 
In single-flowered plants xenogamy is alone possible, but in many- 
flowered ones an insect will carry strange pollen only to the first 
few that are visited -on each plant. As would be expected, gei- 
tonogamy is apparently less beneficial to the species than xenogamy, 
although in plants where both are possible it is usually much more 
frequent. Geitonogamy is of greater advantage to the plant than 



ADJUSTMENT TO TEMPERATURE 



125 



autogamy. This fact explains the large number of allo-autogamous 
plants. 

147. Cross-pollination. The benefits arising from the transfer 
of pollen from one flower to another, as well as the necessity for 
such a transfer in diclinic species, have led to the production of 
numerous devices for bringing it about. These have been developed 




Fig. 32. — Geitonogamy in an aster, Machceranthera aspera, in which pollina- 
tion regularly occurs between neighboring flowers or heads. 

in relation to one of three factors, water, wind, and animals, 
especially insects. With reference to the agent concerned, cross- 
pollinated species are accordingly termed hydrophilous, anemophi- 
lous, or zoophilous. Hydrophilous species may be pollinated 
under water, as in Zostera or Ceratophyllum, or more often the 
pollen is borne on the surface of the water, as in Ruppia, Colli- 



126 



PLANT PHYSIOLOGY AND ECOLOGY 



triche, etc. Anemophilous flowers may lack stigmas, as in the 
gymnosperms, or, more commonly, possess them, as in the angio- 
sperms. Among the latter, five types of flowers are recognized 
with respect to the way in which the pollen is exposed to the wind . 
These are (1) the catkin-bearers, Salix, Populus, Betula, etc.; (2) 
species with hanging flowers, Acer negundo, Rumex, etc.; (3) 
flowers with long slender filaments, Plantago, Graminacece, Cypera- 



fc^MU* . 


A _1 


JliOSI 


• 

: 1 • % 


^Br^?PSr J v 




&jN«8a 


: ( v^ '.j;\r N 


.. ":; - '■> , r 


fmm •ji^W^ja 


- 


jSk r .Ajl5 


WKbpR 


•■ ,7 P: 


^T^jfij 






fn^J\ 


^M 


shLv 1/ 




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Fig. 33. — An orchid, Calypso borealis, with one-flowered scapes, thus making 
xenogamy alone possible. 



cece, etc.; (4) flowers with explosive anthers, Urtica, Parietaria, 
etc.; (5) species with fixed flowers, Typha, Potamogeton, Sparga- 
nium, etc. 

148. Pollination by insects. Zoophilous species may be polli- 
nated by birds, as in Bignonia, Impatiens, and Lonicera; by snails 
as in Ariscema; or even by bats, as in the case of a few tropical 
plants. Plants that are pollinated by insects are by far the most 



ADJUSTMENT TO TEMPERATURE 127 

important, and are very many times more numerous than all 
other zoophilous species. Indeed, more species are pollinated by 
insects than by all other agents combined. It is highly probable 
that all flowers with corollas owe the development of the latter 
to insect-pollination, and this is true also of many species which 
possess sepals alone. The form, color, and fragrance of petaloid 
flowers in particular seem to be due almost wholly to insects. 
They have modified their form to afford landing-places for welcome 
visitors, to enable the latter to find their way quickly to the nectar 
and pollen, and to confuse or repel unwelcome visitors. As would 
be expected, those flowers which have been most strikingly modified, 
such as the mints, figworts, orchids, and many composites, are 
the ones which are dependent for pollination upon insects of a 
certain type. 

Insects are attracted by color or fragrance, and in many flowers 
both methods of attraction occur. Inconspicuous flowers which 
are scentless are nevertheless often visited by insects for the sake 
of the pollen they afford, and the pollen is the real attraction in 
brilliant scentless flowers. These are termed pollen flowers; in 
them a part of the pollen is sacrificed to insure the transfer of the 
remainder. In nectar flowers, nectar constitutes the attraction, 
and the removal of the pollen is incidental. Flowers have pro- 
duced a great many devices to effect the sprinkling or loading of 
insects with pollen, and to insure the deposition of the pollen in 
the proper manner. Furthermore, the opening of the flower at a 
certain time of the day or season is an adaptation to the habits 
of the insects upon which it depends for pollination. 

149. Self-pollination. Autogamy exists as the alternative 
method of pollination in the majority of plants that are regularly 
cross-pollinated. It is the sole method in cleistogamous flowers 
and in those whose size, structure, or position makes them little 
adapted to cross-pollination, or whose habitats present conditions 
unfavorable to the latter, as in the case of many arctic and alpine 
plants. Autogamy is direct in most cleistogamous flowers, and 
in those where contiguity of stamens and pistils, or the position 
of the stamen above the pistil, permits the pollen to fall directly 
upon the stigma. It is indirect when the transfer of pollen is the 
result of movement or growth, as in the majority of autogamous 
plants. Indirect autogamy is brought about by various methods, 
of w T hich the movements of stamens or style, their elongation or 



128 



PLANT PHYSIOLOGY AND ECOLOGY 



contraction, the closing of the perianth, and the falling of the 
corolla are the most frequent. 

Experiment 41. Pollination. Select five flowers of different types, 
representing preferably such divergent forms as the willows, grasses, 
legumes, roses, mints, composites, etc Determine by a series of ob- 




Fig. 34. — Self-pollination in Moneses uniflora by the combined movement 
of pedicel and stamen filaments. (After Kerner.) 

servations the kind of cross or self-pollination which each shows, the 
agent concerned, and the exact manner in which the agent works. In 
those that are pollinated by insects, point out the various devices for 
attraction and transfer. 

150. The period of flowering. The time at which a plant 
opens its flowers and matures its fruits is the result of long-con- 
tinued endeavor on the part of the species to adjust itself to the 
climatic factors of its habitat. Since all the factors are highly 
variable, especially heat, which is the most important in this con- 
nection, the time of flowering varies slightly from year to year. 
In a very abnormal season the variation becomes pronounced. 
Flowering occurs when the amount of growth, which is chiefly 
determined by temperature, reaches a certain stage. The neces- 
sary sum of temperature is reached more slowly in a cool year 



ADJUSTMENT TO TEMPERATURE 



129 



than during a warm one, and the flowers consequently appear 
later. This sum is likewise obtained later as one goes northward 
or ascends mountain ranges, and the time of flowering is cor- 
respondingly delayed. The period during which a species remains in 
flower is similarly t hough less evidently dependent upon temperature. 
In the vast majority of species the period of flowering is largely 
a fixed habit. This is especially evident in many annuals, and 
in species that flower early in the spring. In nearly all cases its 




Fig. 35. — Ephemeral night -blooming flowers of an evening primrose, Pachy- 
lophus hirsutus. The flowers open at sundown; they close at 7-8 a.m. 
the next day and quickly wither. 

position and length have been determined by the necessity of 
rendering it possible for the seeds to mature before the time of 
killing frosts. Species with relatively small and simple fruits, 
such as the grasses and composites, may flower late, while those 
with large or complex fruits, many roses, legumes, etc., usually 
flower much earlier. 

Since flowering is so intimately connected with temperature, 
flowers may be most conveniently classified in accordance with the 
season in wTiich they appear, or in which the period of flowering 



130 



PLANT PHYSIOLOGY AND ECOLOGY 



chiefly falls. A few species, such as the dandelion, bloom through- 
out the growing period, and are termed aianthous or ever-blooming. 
By far the greater number of plants complete their flowering within 
a certain period. Consequently, flowers may be grouped as 
pre vernal, vernal, sestival, and serotinal, corresponding to early 
spring, spring, summer, and autumn. Prevernal flowers are the 
first few that appear before spring has really begun. Vernal bloom- 
ers flower from about the middle of April to the middle of June. 
The sestival period closes about the middle of August, and the 
serotinal period lasts until the time of killing frosts. There is 
naturally no fixed limit for each period, but during each the 
general nature of the vegetation is characteristic. 

151. Time of daily flowering. The time of day at which the 
flowers of each species open, and the life period of a single flower, 




Fig. 36. — Head of an aster, Machcuranthera aspera, showing the position of 
the rays during the day, and at night or in cloudy weather. 

are habits that are more definitely fixed than the seasonal flowering. 
Flowers begin to open as early as 3 a.m. The majority of day- 
bloomers open before 8 a.m., and practically all are open before 
10 a.m. Night-bloomers open between 4 p.m. and 8 p.m., the 
latest usually blooming just at twilight. Many species do not 
close their flowers at all. the latter merely withering and dying 
at the end of the life period. Hemeranthous and nyctanthous 
flowers open and close daily, with the exception of ephemeral 
ones in which the life period is less than one day. These move- 
ments, which are controlled by temperature, ordinarily occur at 
stated times. The dependence upon temperature is so absolute 
that opening or closing may be hastened or delayed by artificial 



ADJUSTMENT TO TEMPERATURE 131 

means. Oftentimes hemeranthous flowers do not open at all 
on unusually cool days, and nyctanthous ones fail to close. The 
majority of nyctanthous species are ephemeral, while only about 
one half of the day-bloomers are of this type. Flowers that open 
and close daily may live for two days only, as in Erigeron and 
Claytonia, or for two weeks, as in Crocus. Flowers that remain 
open are long-lived as a rule. Many of them live for several 
weeks, the maximum period being eighty days for Odontoglossum, 
an orchid. The minimum period, three hours, is found in the 
" flower-of-an-hour, " Hibiscus trionum, which is the most ephem- 
eral of all flowers. The study of the period of flowering and 
of the time of opening and closing constitutes what is commonly 
called phenology, i.e., a study of the phenomena of appearance. 
While this is a fascinating field, its importance at present is 
secondary. 

Experiment 42. Time of flowering. Select ten species which include 
ever- blooming, day-blooming, night-blooming, and ephemeral plants, 
and keep a tabular record during the spring of the time of day when 
the flowers open and close, of the life period of a single flower, of the 
time when the first flower appears, the maximum of the flowering 
period, and its close. 

152. Fructification. The normal consequence of pollination 
is the fertilization of the egg-cell of the ovule, and the development 
of the latter into the seed. In the majority of flowering plants, 
the ovary and its contents are alone concerned in the changes 
of growth which follow fertilization. The ripening ovary of 
this type becomes a simple fruit. In some cases, fertilization is 
followed by a modification of the end of the flower-stalk, producing 
a complex fruit, such as that of the apple, strawberry, blackberry, 
rose, pine, pineapple, Osage orange, etc. Fruits are usually classi- 
fied with respect to their texture as (1) fleshy fruits, including 
stone fruits, and (2) dry fruits, and, with respect to their behavior 
when ripe, as dehiscent or indehiscent. Fleshy and stone fruits 
are indehiscent ; dry fruits may be dehiscent or indehiscent. 

153. Fleshy fruits are characterized by a thickening and 
softening of the wall of the ovary, by which it becomes juicy or 
fleshy. They comprise the berry, drupe, pepo, pome, and such 
multiple fruits as the fig and pineapple. In the berry, e.g., the 
currant, grape, gooseberry, tomato, etc., the whole tissue of the 



132 PLANT PHYSIOLOGY AND ECOLOGY 

ovary is soft. In the drupe or stone fruit, apricot, cherry, peach, 
etc., the outer part of the wall becomes fleshy, while the inner 
hardens into stony tissue. On the other hand, the pepo has 
developed a hard rind upon the outside, while the inside is pulpy. 
The pome of the apple and pear resembles a berry, but the flesh is 
the modified calyx, the pistil being represented by the papery 
core. 

154. Dry fruits are leaf-like, papery, membranous, or hard in 
texture. Indehiscent dry fruits, i.e., those that do not split open at 
maturity, are the achene, the nut, the samara, and the grain. The 
achene is a small seed-like fruit, such as is found in the buttercup, 
strawberry, sunflower, thistle, dandelion, etc . The nut is a fruit which 
possesses a hard stony wall, such as is found in the acorn, hickory, 
and walnut. The samara is an indehiscent fruit provided with a 
wing, e.g., the ash, elm, and maple. A grain is an achene in which 
the wall of the ovary is completely fused with the seed, as in corn, 
wheat, and other grasses. Dehiscent dry fruits, or those that split 
open at maturity, comprise the utricle and the pod, the latter 
being subdivided into the follicle, legume, loment, capsule, silique, 
silicle, and pyxis. A utricle is an achene with a loose, dehiscent 
pericarp, such as is seen in the amaranth and goosefoot. All 
other fruits which split open at maturity are grouped under the 
general term pod. The follicle is a simple pistil which splits 
along the inner suture, e.g., columbine, larkspur, and milkweed. 
The legume is a follicle which opens along both sutures, thus 
splitting into valves, as in the bean, pea, vetch, etc. The legume 
is called a loment when divided into one-seeded joints that separate 
at maturity. The capsule is the pod of a compound pistil. The 
pod of the mustard family, or silique, is two-celled in consequence 
of a false partition which stretches between the valves. The 
silicle is a short, broad silique, such as that of the shepherd 's- 
purse; while the pyxis opens circularly by means of a lid, e.g., 
the plantain and the purslane. 

155. Movements of fruits. The way in which fruits and seeds 
are scattered about is chiefly determined by the nature of the 
fruit, as will be shown in detail under Migration. Certain move- 
ments are also concerned in this to some degree. These so-called 
carpotropic movements result from the bending of peduncle 
or pedicel, by which the position of the fruit is changed, or from 
the growth of the peduncle, by means of which the flower cluster 



ADJUSTMENT TO TEMPERATURE 



133 



is raised. Changes of position are shown in Allium, Campanula, 
Chamcenerium, etc., where the drooping or horizontal flower becomes 
erect after fertilization, largely owing to the action of gravity. 
The upright position appears to promote the ripening of the fruit 




Fig. 37. — Inflorescence of the bluebell, Campanula petiolata, in which the 
bud is erect, while the flower and fruit are turned downward by the 
movement of the pedicel. 

and to place the seeds in a more advantageous position for dis- 
semination. The elongation of the scape in consequence of 
growth after flowering, such as occurs in the dandelion and other 
stemless composites, seems to be for the purpose of lifting the 
achenes above the surrounding plants, in order to increase the 
chances that they will be borne away by the wind. 



Experiment 43. Kinds of fruits. Since relatively few fruits mature 
in the spring, the study of the kinds of fruits and their relation to migra- 
tion should be made early in the autumn. The number of fruits at this 
time is very large, and the opportunities for observation unusually 
favorable. A field trip should be made through prairie and woodland in 
early fall, the various kinds of fruits noted, and the species grouped 



134 



PLANT PHYSIOLOGY AND ECOLOGY 



accordingly. This work may well be combined with the study of dis- 
semination and migration indicated on a later page. 




Fig. 38. — Umbels of a wild onion, Allium recurvatum. The buds and 
flowers droop, but the pedicels begin to curve upward after fertiliza- 
tion, and the capsules become erect. 

Grow plants of Campanula rotundifolia or Chamcenerium angusti- 
folium. Trace in detail the movements of flower and capsule, and 
sketch the characteristic positions. 



CHAPTER VI 
ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 

156. The relation of the plant to gravity. Gravity differs from 
all the factors previously considered in being constant and in 
affecting all plants essentially alike. Although it occurs in every 
habitat, exerting a profound control upon the relation of root, 
stem, leaves, and flowers, no essential differences between stemmed 
plants arise from its action. This is an immediate result of its 
constancy, and consequently under normal conditions gravity 
has no power to produce modification. In fact, the control exerted 
by it is stabilizing rather than modifying. The first terrestrial 
plants in all probability possessed flat thalloid bodies. Through 
the action of two opposite media, air and soil water, the thallus 
became differentiated, a change further emphasized by the different 
light intensity at the leaf and the root surface. Any tendency 
upon the part of the leaf surface to grow upward, or to become 
upright, tended to increase the light energy available, and the 
downward growth of the hair-like roots increased the water supply. 
Plants that thus became polarized were placed at a great advantage 
over the thalloid forms. They were doubtless the ancestors of 
the vascular plants. It does not seem probable that gravity 
played a considerable part in producing the polarity shown by 
stemmed plants. As this habit became more and more fixed, 
however, it necessarily acquired a constant relation to gravity. 
The roots grew downward in line with the pull exerted by it, while 
stems grew constantly in opposition to it. After countless genera- 
tions, the relation has become so firmly established that the control 
exercised by gravity is much greater than that of light. 

157. Geotropism. The relations of plants to gravity are com- 
prised in the term geotropism. The actual bending or turning of 
an organ in response to gravity is evident only when the normal 

135 



136 



PLANT PHYSIOLOGY AND ECOLOGY 



position is disturbed. Geotropism is equally characteristic of the 
normal position, since the latter is maintained only in consequence 
of it. Stems are negatively geotropic, i.e., they grow away from 




Fig. 39. — Inflorescence of the fireweed, Chamcenerium angustifolium, show- 
ing the movements of the pedicels and the position of bud, flower, and 

fruit. 

the attraction of gravity. Roots are positively geotropic, i.e., 
they grow in accordance with the pull of gravity. Leaves of the 
dorsiventral type place themselves more or less nearly at right 
angles to the stem and are hence termed diageotropic. In addition 



ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 137 

to maintaining this angle, loaves also tend to keep their surfaces 
horizontal. Flowers and fruits are sometimes diageotropic also. 
In certain species they change their relation to gravity, as is 
regularly the case in anthotropic and carpotropic movements. 
For example, the bud of Chamcenerium angustifolium is positively 
geotropic, the flower diageotropic, and the fruit negatively geo- 
tropic. Branches of the stem and root are usually diageotropic, 
though they are capable of changing this relation in considerable 
measure. 

158. Cause and reaction. The exact w r ay in which the stimulus 
of gravity is perceived by the plant is not known with certainty. 




Fig. 40. — A young plant of Fuchsia sp., showing the effect upon leaf posi- 
tion when the control exerted by gravity is destroyed by growing the 
plant under the action of centrifugal force. 

It has been suggested that the fall of starch grains or other in- 
clusions to one side when the position of the plant is changed 
sets up a stimulus in the protoplasm, but this does not explain 
all cases of geotropism. It seems more probable that protoplasm 



138 



PLANT PHYSIOLOGY AND ECOLOGY 



f 



possesses a specific sensibility to gravity, just as it does to light. 
When a change in the position of the plant occurs, the normal rela- 
tion of the cytoplasm to gravity is modified, and changes are set 
up in it that tend to restore the normal. The perception of the 
stimulus of gravity by the root and the stem takes place largely 
in the meristem of the tip. In the root the sensory zone is scarcely 
more than a millimeter wide, comprising the tip alone. This zone 
in the stem is less restricted, the entire meristem of the apex being 
sensitive. In addition, the cells of the cortical parenchyma, and 
sometimes those of the pith, are capable of perceiving the stimulus. 
In the case of leaves and flowers, it is probable that the power of 
perception resides in all the living cells. 

The length of time for which a plant must be changed from its 

normal position before a response to gravity becomes evident is 

termed the reaction time. It is a curious 

A 

fact that only a part of this time is neces- 
sary for the perception of the stimulus. If a 
plant is put in a horizontal position for a 

r - r — quarter of an hour, and then placed upright, 

|^ geotropic curvature will still occur. Con- 

o sequently, reaction time consists of two 
periods, one necessary for perception, the 
other for reaction. 

159. Region of curvature. The geotropic 
curvature of the root is effected in a zone 
scarcely wider than the sensory zone and 
lying just behind it. The manner in which 
the stimulus is transmitted from the one to 
the other is unknown. The actual bending 
is due to the elongation of the cells upon 
the upper or converse side, and is accom- 
panied by the compression of those upon the 
concave surface. The region of curvature in 
the stem is much more extensive than that 
of the root. The reason for this difference is 
apparently to be found in the fact that the 
major portion of the root is held firmly by 
the soil, while the stem is free to curve. The curvature of the 
stem is first apparent in the region of rapid growth just below 
the tip.- It travels downward in such a manner that, as it ap- 




3 45 



°t 

Fig. 41 . — The region of 
curvature in the pri- 
mary root of the 
horse-bean, Vicia 
{aba. In A, the root 
was marked into five 
areas of 2 mm. each; 
B shows the growth 
and curvature at the 
end of two hours, and 
C, at the end of 
twenty-three hours. 
(After Sachs.) 



ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 139 

proaches the base, the tip is carried beyond the vertical. Finally, 
the tip swings back and is held in the normal position by the 
fixing of the curvature in the base of the stem. In leaves the 
region of curvature normally lies in the petiole, but if the leaf is 
reversed and held firmly in some part of the blade, the free area 
will attempt to twist into the normal position. In the case of 
flowers, curvature normally takes place in the peduncle. 

160. Ecological significance of geotropism. Apart from the 
fundamental action of gravity in maintaining the position and 
form of plants, geotropism is of great value to plants in remedying 
the effects of accidents due to wind, snow, animals, etc. In many 
trees, especially conifers, geotropism brings about the replacement 
of a broken or injured apex by one of the branches. Trees that 
have been blown dowm sometimes regain an upright position, in 
part at least, by means of geotropism, and they often convert the 
branches of one side into upright stems. Herbs that have been 
blown down by the wind or trampled by animals regularly regain 
an upright position, at least in the new grow T th. Inflorescences, 
flowers, and leaves that have been bent or caught in a mass of 
leaves and branches, turn and twist to resume their normal position. 
One not infrequently finds a stem hanging in such fashion that the 
usual relations of the flowers are exactly reversed, and in conse- 
quence the flowers, as well as the fruits, have curved about to take 
their normal position. Finally, geotropism is a powerful factor 
in the successful germination of seeds in nature by virtue of its 
action in carrying the radicle into the soil. 

Experiment 44. Geotropism. Plant sunflower seeds in a pot, placing 
some flatwise, and others with either end downward. Cover the pot 
with a wire netting to hold the soil in position, and invert it upon a 
tripod, using a bell-glass to cover both. Explain the behavior of the 
seedlings, and note the relation of the curvature to the original position 
of the seed. 

Transplant a sunflower seedling to each of six 2-inch pots. After 
they are well established, place one in darkness in a horizontal position, 
and one in the same position in light. For four successive mornings 
put one of the remaining pots in a similar position. In the case of the 
last two pots, restore the last plant to its normal relation after a half- 
hour and note results. Remove the seedlings from the pot and sketch 
them in order, showing the position of stem and root. Compare with 
the plant grown in darkness Restore the last seedling to an upright 



140 



PLANT PHYSIOLOGY AND ECOLOGY 



position, and note the time that elapses before it reaches the perpen- 
dicular. 

Select three plants of Chamcenerium that are beginning to flower. 
Reverse a flower and leaf of one, fixing the base of the stalk so that the 




wm 




Fig. 42. — A flowering branch of the fireweed, Chamcenerium angustifolium, 
accidentally broken and inverted. The pedicels are curving to place 
the flowers and fruits in the usual position. 



blade and flower are free to turn. Turn the other plants upside down, 
placing one in darkness. Make sketches of the resulting changes, and 
explain them. 



ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 141 

161. Response to contact. Many climbing plants have 
developed organs called tendrils, by means of which they cling to 
their supports. Tendrils are characteristic of climbers, Micram- 
pelis, Sicyos, Vitis, etc., in contrast to the twiners, such as the 
bind weal, morning-glory, etc. Tendrils are specialized branches 
or leaf parts which have become especially sensitive to contact. 
Similar sensibility in a less degree seems likewise to occur in the 
stems of twining plants. The growing point of roots is also sensitive 
to contact, and is consequently enabled to pass a hard substance 
which lies in its path. 

The sensory area of a tendril is usually restricted, though in 
a few cases it extends over the major portion of it. As a rule, 
the basal portion is scarcely or not at all sensitive, and the sensory 
area is confined to the concave side of the curved or hooked tip. 
A few tendrils respond to contact upon any side, while others respond 
only to lateral contact, in addition to that upon the lower side. 
Generally speaking, they are not sensitive when either very 
young or very old, but react only while they are growing and 
showing circumnutation, i.e., the constant movement of the 
tip in a circular manner. The value of circumnutation in bringing 
the tip in contact with a possible support is evident. When 
the sensory area is brought into contact with a stimulus of the 
proper kind, the tip begins to curve. The curving may begin in 
less than a minute after the contact, or several hours may elapse 
before it becomes evident. Sensitive tendrils respond to a 
momentary slight touch, but for the majority a stronger stimulus 
is necessary. The size and surface of the support have much to 
do with the presence and nature of the response. 

In certain plants the stimulus affects only the point of contact, 
but in the majority of cases it is also transmitted, and results in 
the spiral coiling so characteristic of many tendrils. This reaction 
usually begins near the sensitive tip and travels toward the base. 
Its advantage is evident in that it lifts the stem and serves to hold 
it firmly, but not so rigidly that it may be easily torn away. The 
curvature which produces attachment as well as the spiral coil 
is the result of unequal growth. The cells of the sensory side 
either do not elongate at all after contact, or they do so more 
slowly than the cells of the opposite side. The coil becomes 
fixed in consequence of the development of the tissues into a 
more permanent form. 



142 PLANT PHYSIOLOGY AND ECOLOGY 

Experiment 45. The behavior of tendrils. Experiment with the ten- 
drils of Cucurbita or Micrampelis to determine the sensory zone as well 
as the period during which the tendril is sensitive. Find the length 
of time necessary for contact to produce curvature in the tip, and for 
the transmission of the stimulus as shown by the formation of the coil. 
Note the behavior of tendrils that fail to reach a support, and of re- 
cently formed coils from which the support is removed. Sketch a ten- 
dril in various stages. Ascertain by sectioning the behavior of the cells 
on the convex and concave surfaces. 

162. Response to shock. While all protoplasm possesses in 
some degree the power of response to mechanical shock, this 
reaction is readily seen only in moving or streaming protoplasm, 
and in certain specialized organs or plants, such as the stamens 
of some cacti, and the leaves and stems of sensitive plants. The 
best illustration of response to shock is afforded by the common 
sensitive plant, Mimosa pudica. The normal reaction consists 
of the folding of the leaflets and the drooping of the whole leaf 
at the point of union between petiole and stem. A slight shock 
merely causes the leaflets to close, and it is quite possible to touch 
the leaves so lightly that it does not constitute a stimulus. At 
least, no visible reaction takes place. The vigor of the plant also 
has much to do with the response. The healthy leaves of plants 
that had at one time been subjected to drouth and cold responded 
but feebly to heavy blows, merely moving the leaflets slightly, 
while those of a normal plant reacted fully to a gentle touch. 

The perception of shock by the sensitive plant is scarcely if at 
all localized. Nearly all the epidermal cells of stem and leaf 
have the power of perception, except certain cells of the base of 
the petiole, i.e., the pulvinus. When the leaf or stem is struck 
vigorously, the stimulus probably acts directly upon the pulvinus, 
the leaflets folding, and the petiole drooping almost at once. The 
perceptive power of any leaflet may be readily shown by striking 
it gently. The impulse travels down the axis, closing the leaflets 
as it goes, until it reaches the pulvinus, when the whole leaf droops. 
The manner in which the stimulus is transmitted is not certainly 
known. It is supposed to take place through rows of turgid 
tubular cells, which lie near the bundles, by means of disturbances 
in the cell-sap. 

The movements of leaflets and leaves are due to changes in 
the pulvinus, a mass of swollen tissue at the base of the petioles 



ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 143 

of leaflet and leaf. The pulvinus consists chiefly of turgid paren- 
chyma, surrounding a nbrovascular bundle sheathed in collen- 
chyma. In consequence of a stimulus, the protoplasm of the 
cells upon the lower side of the leaf pulvinus contracts, forcing 
a small amount of cell-sap out into the air spaces. This destroys 
the turgidity of the lower cortex, and at the same time shortens 
the cells. This releases the tension upon the cells of the upper 
side, and allows them to expand, thus causing the leaf to bend 
downwards. This action is emphasized by the pull of the leaf 
itself. In the pulvini of the leaflets, the contraction must occur 
upon the upper side, allowing the lower cortex to expand, and to 
raise the leaflets in direct opposition to gravity. After a response, 
the contractile cells gradually absorb the excreted w r ater and 
regain their turgidity, thus restoring leaf and leaflets to their 
normal position. 

Experiment 46. Response to shock. Grow several plants of Mimosa. 
Make various experiments to ascertain what regions are sensitive, as 
well as the strength of stimulus necessary to produce a response. Strike 
the end leaflets of two leaves at the same time, one forcibly, the other 
gently, and note the time necessary for the transmission of the impulse 
to the pulvinus of each leaf. Note when leaves regain the normal posi- 
tion, the effect of repeated blows upon a leaf that has reacted, and 
how long a time it requires to resume its normal condition. 



CHAPTER VII 
ADAPTATION TO WATER 

163. The relation of structures to water. The functional 
responses of the plant to water content and humidity may produce 
modification of form, of structure, or of both. This may take 
place in root, stem, or leaf, or, under intense conditions, may 
occur in all of these. Modification is greatest in the leaf as the 
organ of greatest activity, and of greatest exposure of surface to 
the changing factors in the air. It operates upon the stem in 
the degree that the latter carries on the functions of a leaf. The 
root is changed least, owing to the greater uniformity of conditions 
in the soil, as well as to its fewer activities. Under extreme con- 
ditions, either organ may be lost. As would be expected, the 
root is frequently lost when it is no longer needed for absorption, 
and the leaf when the water supply is reduced to a minimum. 

164. Adaptation to a small water supply. A low water supply 
threatens the functions of the plant, and consequently its growth 
and existence, whenever the water loss is increased. The effect 
of a deficiency in the supply must be met by changes in structure 
which decrease the demand arising from water loss, or by those 
that increase the supply by adding to the absorption or storage 
capacity of the root. These changes affect the form and size of 
the various organs as well as their structure. Modification in the 
form and size of leaf or stem lessens transpiration by reducing 
the amount of surface exposed to the air. Changes of structure, 
on the other hand, bring about the protection of epidermal cells 
and stomata, as well as the internal tissues, from the factors that 
promote transpiration. They also forestall the effects of excessive 
water loss by storing water in specialized cells or tissues against 
periods of low humidity. In a few extreme cases the epidermis 
may be modified for absorbing water vapor from the air. 

144 



ADAPTATION TO WATER 145 

The modifications of the plant which serve to decrease water 
loss may be grouped under the following heads: (1) position of 
the leaf; (2) rolling of the leaf; (3) reduction of the leaf or stem 
surface; (4) epidermal modifications, (a) of epidermal cells, (b) 
of stomata; (5) changes in the chlorenchym. 

165. Decrease of water loss through leaf position. Horizontal 
leaves, as a rule, transpire more than those which take a vertical 
or oblique position. Since the light energy is greatest when the 
sun is highest, those leaves transpire least which make the smallest 
angle with the rays of the sun during the middle of the day. A 
leaf at right angles to the rays of the sun receives almost ten 
times as much light and heat upon the same surface as one placed 



^^*y 



Fig. 43. — Plants of the dogbane, Apocynum androscbmifolium. The hori- 
zontal position of the leaves at night or early morning changes toward 
midday to the vertical, thus protecting the leaves from the direct rays 
of the sun as well as from the strong radiation from the gravel soil. 

at an angle of 10°. Reduction of water loss by means of the 
vertical or oblique position of the leaves is a frequent occurrence 
in the erect or hanging leaves of many tropical trees. A similar 
means is found in " compass plants," such as Silphium laciniatum, 
Lactuca scariola, etc., and in all species with more or less erect, 
hanging, or equitant leaves. The effect, however, is just opposite 
in the sunflower and other heliotropic species, since the turning 
of the crown tends to maintain a position at right angles to the 
rays. In the case of plants that grow in mats, the aggregation of 
stems brings about the mutual protection of the leaves. In addi- 
tion, mats often have erect or oblique leaves. 

166. Decrease through the rolling of the leaf. In a large num- 
ber of plants, the amount of leaf surface exposed to dry air is re- 
duced by the rolling or folding of the leaf. Rolling occurs in many 



146 



PLANT PHYSIOLOGY AND ECOLOGY 



plants merely as a temporary compensation. Water loss takes 
place more rapidly, as a rule, from the surface bearing the larger 
number of stomata. In consequence, the edges are usually rolled 
up with the lower side inward, chiefly by reason of the greater 
turgidity of the upper. The furrowed leaves of monocotyledons, 
especially the grasses, are well adapted to changes of this nature. 
The leaves of many grasses and heath plants are permanently 




Fig. 44. — Sun and shade forms of Wagnera stellata. The leaves of the sun 
form are folded or rolled together, while the shade leaves are flat. 

rolled or folded. In these the protection against drouth is very 
effective. It arises not only from the reduction of surface, but. 
also from the fact that the stomata lie in a chamber that is perma- 
nently and more or less completely closed. Many mosses roll and 
twist their leaves when threatened by drouth, but in these the 
rolling merely reduces the leaf surface exposed. 

167. Reduction of leaf or stem. Plants reduce their surface, 
and thereby the amount of transpiration, by decreasing the number 



ADAPTATION TO W.YTKIt 



147 



of leaves, by reducing the size of each leaf, or by a change in its 
form. Among herbaceous plants, a decrease in the size or a 
change in the shape of the stem brings about a similar result. 
In extreme cases of reduction, the leaves are completely lost, and 
in some instances the same fate overtakes the stem. Such a 
marked decrease in the amount of surface exposed is found only 
in intense xerophytes, though it occurs in all deciduous trees and 
shrubs as a temporary adaptation. Changes in leaf form regularly 
produce a decrease of surface. The scale, the linear or cylindrical 
leaf, and the succulent leaf are the most striking examples of 
reduced leaf forms. Lobed or divided leaves usually show a 
tendency to reduce the size of the lobes or divisions when they 
are grown under drier conditions. 

168. Changes of the epidermal cells. The cells of the epidermis 
are protected against evaporation by a coating of wax or other 




Fig. 45. — Portion of a cross-section of a leaf of the century-plant, Agave 
americana. The outer wall of the epiderm is modified to form a very 
thick cuticle, and the stomata are sunken below the surface. 



material, by means of a thickened outer wall or cuticle, or by the 
development of hairs. The first two sometimes occur upon the 
same leaf, but the formation of a hairy covering is usually precluded 



148 PLANT PHYSIOLOGY AND ECOLOGY 

by the presence of either of these. It often occurs, however, on 
the lower side of a leaf that is cutinized above. Excretions of 
wax or of salts render the epidermis highly impermeable, and 
correspondingly reduce the loss of water through the epidermal 
cells. The thickening of the outer wall of the epidermis to form 
a cuticle is the most perfect of all devices for decreasing permea- 
bility, and thus reducing transpiration. In many desert plants 
the greatly thickened cuticle completely prevents all transpiration 
except that which occurs through the stomata. In these the 
cuticle is also regularly developed in such a way as to protect 
the guard-cells. Some species have an epidermis that comprises 
two or more layers of cells. While this is an effective protection 
against water loss, it is not frequent. 

A coating of hairs decreases transpiration by screening the 
epidermis so that the amount of light and heat is diminished, 
and the access and movement of dry air impeded. A few scattered 
hairs are of little or no value for this purpose, but a uniform com- 
pact layer is of the greatest service, since it protects the stomatal 
openings as well as the epidermal cells. Hairs are of the most 
various sizes and forms, but all hairy coverings serve the same 
purpose, even when they are primarily for water storage, as in a 
few plants. The fact that hairs protect the stomata as well as 
the epidermal cells explains the occurrence of a hairy covering 
on the lower surface, even when it is absent from the more exposed 
upper side. In some cases, hairs are developed only to serve as 
screens to the stomata. 

169. Modifications of the stomata. Since the great bulk of the 
water lost under ordinary conditions passes through the stomata, 
the changes of the latter are of the utmost importance in reducing 
transpiration. Their modifications for this purpose are many, 
but practically all of them are concerned with number or position. 
Species growing in dry places have fewer stomata to the same 
leaf area than those in moist habitats. The number on both sur- 
faces decreases as the danger of excessive water loss increases. 
The decrease is usually more rapid upon the upper surface, which 
finally loses its stomata entirely. Stomata are usually more 
numerous on the less exposed or lower surface of the leaf. Excep- 
tions occur in many shade plants where the exposure of the two 
surfaces is equal, and in aquatic plants, in which water loss is 
beneficial instead of harmful. The change in the number of 



ADAPTATION TO WATER 



149 



stomata is well illustrated by Ranunculus sceleratus, a species of 
wet places, in which the stomata are more abundant upon the 
upper surface 4 . Plants grown in water with the leaves floating, 
and in soils containing 40%, 30%, 15%, and 10% of water, showed 
respectively the following results with respect to the stomata of 
the two surfaces: upper 20, lower 0; upper 18, lower 11; upper 11, 
lower 8; upper 10, lower 6. 

Reduction in the number of stomata gives sufficient protection 
only under moderate conditions of dryness. Where dryness is 
intense, the guard-cells are usually found sunken below the epi- 
dermis, either singly or in groups. Sunken stomata are generally 




Fig. 46. — Upper epiderm of a submerged (1) and an aerial leaf (2) of a plant 
of Callitriche bifida, showing the decrease in the number of stomata. 

found at the bottom of chimney-like openings, which are some- 
times almost completely closed above. When the stomata are 
sunken in groups, the cavities are commonly filled with protective 
hairs, or closed by them. In both cases the protection is very 
effective. The guard-cells are screened from the intense action 
of light and heat, and from the dry air. The rays of the sun can 
enter the chimney-shaped chambers only for a few minutes each 
day, and are practically excluded from the stomatal hollows, which 
are filled with hairs. The influence of dry winds is likewise almost 
wholly eliminated. This is true in a less degree for stomata which 
are arranged in furrows protected by intervening ridges. The 
cuticle often forms valve-like projections upon the guard-cells 



150 



PLANT PHYSIOLOGY AND ECOLOGY 



or above them, which serve to reduce the size of the opening. 
In some cases, moreover, the size of the pore formed by the two 
guard -cells is permanently reduced. In a few plants the effect 
of intense drouth is almost completely prevented by closing the 
pore by means of a waxy excretion. 

170. Changes in the chlorenchym. The rapidity with which 
water escapes from the tissue of the leaf is largely determined by 
the size and number of the air passages. Water-laden air reaches 
the stomata most easily when the air spaces are large and con- 
tinuous, and least readily when they are small and scattered. 
Consequently, leaves exposed to the danger of excessive water 




Fig. 47. — Leaf of a plains species, Bahia dissecta, in which the chlorenchym 
consists entirely of palisade tissue. 



loss usually have the size of the air spaces reduced, and especially 
the size of the passages that connect them with the pores of the 
stomata. An increase of palisade tissue reduces many of the air 



ADAPTATION TO WATER 151 

spaces to mere lines, and thereby greatly decreases the amount 
of transpiration. The conversion of sponge tissue into palisade 
tissue further diminishes water loss by placing the chloroplasts 
in such a position that they mutually shield each other, and thus 
reduce the transpiration caused by light. The development of 
layers or masses of stone fibers, or sclereids, beneath the epidermis, 
though primarily for support, likewise hinders the escape of 
moisture. Such modifications are frequent in needle leaves, 
especially those of evergreen trees, pines, spruces, etc. The 
cell-sap sometimes plays an important part, by holding water in 
cells which have a high salt content, or contain more or less 
mucilage. 

171. Increase and storage of water supply. The amount of 
water supplied to the leaves by the roots can be increased only 
by increasing the area of the absorbing surface, or by changing 
its location. The production of more root hairs, accompanied 
by the growth and branching of the roots, is the usual response of 
plants to moderate drouth. Plants of dry habitats increase their 
absorption by extending the absorbing surfaces of the root into 
the deeper portions of the soil, as well as by their branching within 
this area. Water loss from the root surfaces in contact with the 
dry upper soil is prevented by means of a well-developed cortex. 
For these reasons xerophytes are often characterized by the 
possession of tap roots. Some plants, chiefly epiphytes, absorb 
rain water and dew by means of their leaves. A few desert plants 
seem able to condense the moisture of the air by means of hygro- 
scopic salts, or in other ways, and to absorb it through the epi- 
dermis of the leaf. For all plants with roots, however, the amount 
of absorption by stem or leaf is inconsiderable, and can play no 
important part in increasing water supply. 

There is a limit to the increase of water supply by the extension 
of the root surface. In consequence, many xerophytes have 
developed structures for storing water. Modifications for water 
storage are occasionally found in roots and stems, such as those 
of many fleshy plants. Storage devices occur chiefly in the leaves, 
where they are of great importance. They increase the water 
supply by storing the surplus of absorbed water against a time of 
need. Moreover, they often retain the stored water with great tenac- 
ity, and thus tend to offset the pull exerted by evaporation. 
The epidermis is frequently modified to form reservoirs for water. 



152 



PLANT PHYSIOLOGY AND ECOLOGY 



These may consist of the epidermal cells proper , of layers of water 
cells just below the epidermis, or of swollen cells found upon its 
surface. Cells of the chlorenchym are often transformed into 
large clear water cells, which may be scattered singly or arranged 
in groups. The groups of water cells are sometimes scattered, 
but they usually occur in transverse bands, or in horizontal layers. 
Such layers lie between the palisade and sponge tissues, and 
connect the bundles. A few plants possess tracheid-like cells 
which serve to store water. In the case of succulent leaves, 
practically the whole chlorenchym is used for storing water. 
Such leaves retain their water tenaciously by virtue of mucilage 
or other substances. 

172. Adaptation to excessive water supply. Plants which 
grow in water but have their leaves exposed undergo changes 




Fig. 48. — Cross-sections of floating (5a), submerged (56), and deeply sub- 
merged leaves (5c) of Sparganium angustifolium. The palisade tissue 
of the floating leaf is replaced by a single row of sponge cells in the 
submerged one and the air-passages correspondingly increased. The 
deeply submerged leaf lacks palisade tissue and the air-passages are 
much reduced. 

that increase the water loss and decrease the water supply. The 
absorbing surface is much reduced by the uniform lack of root 
hairs, and the relatively small development of roots. In a few 
extreme cases the roots become mere vestiges or are entirely 
wanting. The leaves of water plants show a marked tendency 
to increase the exposed surface. This is clearly shown by the 



ADAPTATION TO WATER 153 

experiments with Ranunculus seder atus, in which the latter was 
grown under varying conditions of water content. The leaves 
of the mud and floating forms were found to be larger than those 
of the drier soils, but they had changed little or not at all in thick- 
The lobing of the Leaves was also found to be reduced, or 
the lobes often came to overlap. Water plants rarely show any 
modifications of epidermis or stomata, which could serve to hinder 
transpiration. Stomata are usually more numerous upon the 
upper surface, where they are completely exposed. In the same 
species their number is greater in the forms grown in wet places. 
The air spaces are extremely large, and, in connection with the 
abundant stomata, permit of very rapid transpiration. The 
increase in the amount of air space is accompanied by a reduction 
of palisade tissue, and a decided increase in the sponge tissue. 
The result is to expose the chloroplasts more completely to the 
sunlight, and to augment the consequent loss of water. The 
complete absence of storage tissues is a further indication that 
the leaves of water plants are adapted to promote w T ater loss. 

Experiment 47. Experimental adaptation to water. The most satis- 
factory plants for experiments in adaptation are found in plastic species, 
such as Ranunculus sceleratus, and the so-called heterophyllous ones, 
e.g., Ranunculus delphinifolius, Roripa americana, etc. These are 
amphibious, i.e., capable of growing in water or on land, and are con- 
sequently able to undergo adaptation in both directions. Seeds of 
these plants may be collected in the field, or the plants may be trans- 
ferred to the greenhouse and allowed to mature there. Ranunculus 
sceleratus is especially suited to work of this sort, since it grows and 
reproduces with the greatest readiness in the plant house. 

The seeds are germinated under the usual conditions beneath glass 
or sphagnum. After the plantlets have developed four or five leaves, 
thirty or forty are transplanted into 2-inch pots. The transfer to 4-inch 
pots is made about the time the leaves reach ten or twelve in number. 
The plants are still kept under uniform conditions for a few days until 
they are well established. They are then subjected to different con- 
ditions, ranging from soil as dry as possible for growth to submergence 
beneath the water. Four pots are watered in such amount that the 
plants are just able to make a slight growth. The proper amount can 
only be found by trial: it is usually from 25-50 cc. per day. A second 
series is watered with twice this amount, approximately 100 cc, and 
a third one with 200 cc. for each plant. The fourth series is grown in 
mud covered with a thin layer of water, and the fifth in water, the level 



154 PLANT PHYSIOLOGY AND ECOLOGY 

of which is raised from time to time so that the leaves are kept floating. 
These two series are handled most conveniently if the plants are grown 
in a deep tub or a half -barrel. The last series should be grown in a large 
barrel or in a deep box, one side of which is replaced by glass. Six 
inches of soil are placed in the bottom and a faucet is inserted in the 
side just above the soil. This is to aid in the aeration of the water 
from time to time, as well as to make it possible to draw it off readily 
in case it becomes stagnant. The water level is kept just above the 
leaves as they stretch up. In case the leaves begin to turn yellow, they 
are allowed to float on the surface until they regain the normal color. 

All the series should be grown in the same house where heat, light, 
humidity, etc., are the same. In this event all the modifications ob- 
tained can be referred with certainty to water content as the cause. 
In the three series, mud, floating, and submerged, the soil is saturated 
and it is unnecessary to measure the water content. The latter should 
be determined several times during the course of the experiment for the 
three series in soil. The growth and the behavior of the plants of the 
various series should be carefully followed and compared throughout 
the experiment. In connection with growth, it is sometimes desirable 
to compare the soil temperatures with those of the water. When the 
instrument is accessible, a water photometer should be used to deter- 
mine the light intensity for submerged leaves, and for the under side of 
floating ones. 

As the plants come into full flower, an individual from each series 
should be carefully sketched, or photographed, to a fixed scale showing 
the branching of roots and stem, leaf and flower production, etc. A 
typical leaf should be drawn in like manner, and its area and thickness 
carefully determined. The relative water loss should be determined by 
placing a leaf from each series in a vial of water in the manner already 
employed in a previous experiment. If the number of leaves on each 
plant is counted, this will make it possible to approximate the evapo- 
ration for the whole plant. Careful counts of stomata should be made 
for both surfaces of a representative leaf of each series, the actual 
transpiring surfaces, i.e., the air spaces, estimated, and a table of 
comparisons made. Finally, microtome sections should be made of 
similar leaves and a segment of each sketched in the proper sequence 
to show differences of thickness, structure, etc. 

173. Types of plant body. The plant regularly bears the 
impress of its habitat in the form or structure of some or all of 
its organs. This impress is usually recognizable at a glance, and 
can be referred at once to water or light stimuli. Since it is the 
product of the present habitat, the plant which bears it is called 



ADAPTATION TO WATER 155 

a habitat-form, or ccad. Thus, there are water ecads, and light 
ecads, i.e., plants whose character has been determined by adapta- 
tion to water, or to light. The same species may show several 
ecads, in case it grows in habitats sufficiently different, a fact well 
illustrated by the various forms of Ranunculus sceleratus. In 
addition to the impress which a species owes to the water or light 
of its habitat, it possesses other characteristic features, which can 
be referred only indirectly or not at all to these. Such are the 
forms termed trees, shrubs, grasses, etc. These are characteristic of 
great areas of vegetation, and are hence termed vegetation forms 
or phyads. The causes that produce them lie hidden in the history 
of each species, and at present they can only be grouped with 
respect to form. 

174. Types produced by adaptation to water. Plants which 
grow habitually where the water supply is low show one or more 
of the characteristic modifications due to the latter. They are 
consequently termed dry-land plants or xerophytes. Those found 
in habitats with an excessive water supply show corresponding 
modifications, and are called water plants or hydrophytes. Xero- 
phytes and hydrophytes represent more or less extreme conditions 
of habitat and structure. Habitats which are neither dry nor 
wet produce plants intermediate between these two types. Such 
intermediate plants, or mesophytes, show no characteristic modi- 
fications. As a rule, definite structures for increasing water 
supply or decreasing water loss are either slightly developed or 
completely absent. As would be expected, certain mesophytes 
approach the xerophytes, while others are more or less hydrophytic 
in nature. The plants of forests, meadows, prairies, and cultivated 
fields are usually mesophytes. Those of high prairies, tablelands, 
plains, sandhills, deserts, alpine peaks, etc., are xerophytes, and 
the dwellers in wet meadows, swamps, ponds, streams, and other 
bodies of water are hydrophytes. Generally speaking, xerophytes 
grow in dry soils, mesophytes in moist soils, and hydrophytes in 
wet soil or in water. Partly on account of the influence of humidity, 
and partly because many habitats shade very gradually into each 
other, it is impossible to establish an absolute correspondence 
between each group and the water content. Xerophytes commonly 
grow in soils whose holard is less than 15%, and with a chresard 
of 5-10%, while hydrophytes grow in saturated soils or in water. 
Sands and gravels are saturated at about 20%, and hence contain 



156 PLANT PHYSIOLOGY AND ECOLOGY 

hydrophytes at a percentage which in a finer soil suffices only for 
mesophytes. This is largely due to differences in the amount 
of available water. Such differences serve also to explain the 
apparent lack of correspondence in alkaline soils, etc. 

175. General features of xerophytes. Most xerophytes possess 
a deep-seated root system, which is able to draw water from the 
lower moist soil, and to conserve it from loss in the dry upper 
layers. Reservoirs for water storage are rarely developed in the 
root. The stem usually shows modifications more or less similar 
to those of the leaf. The stem is often reduced and sometimes 
disappears, though not all stemless plants are xerophytes. The 
stem is most modified as a rule when the leaves are greatly reduced 
or absent. 

The organ which is most strikingly modified in xerophytes is 
the leaf. This exhibits a large number of variations in size, form, 
texture, and structure. Several of these are often combined in the 
same leaf, though as a rule one alone is characteristic. The most 
satisfactory grouping of xerophytes is upon the basis of the leaf, 
since it is the organ most directly affected. Hence, those plants 
on which the leaves are present and properly modified may be 
termed leaf xerophytes, and those in which the stem has been 
modified in the absence of the leaves, stem xerophytes. Various 
groups of xerophytes have also been distinguished with respect 
to certain factors which reduce the water supply. Thus, species 
of saline and alkaline soils have been termed halophytes or salt 
plants, those of arctic habitats, polar xerophytes, and those of 
many bogs and swamps, bog xerophytes. The latter are probably 
not xerophytes at all, while the other two show no essential 
differences from the ordinary types. They are due to the lack of 
available water, and exhibit most of the common xerophytic 
modifications. 

176. Types of leaf xerophytes. In these, adaptation has acted 
primarily upon the leaf, while the stem has remained normal, or 
has changed but slightly in most instances. In some cases the 
leaves have been reduced to scales, but even then they persist 
throughout the growing season, and continue to take the primary 
part in photosynthesis. Leaf xerophytes may be arranged in 
groups based upon the form of the leaf or its structure. Since the 
same leaf sometimes shows two or more structural modifications, 
a grouping with respect to form is the most satisfactory. The 



ADAPTATION TO WATER 157 

following types may be distinguished: (1) the normal form; (2) 
the succulent form; (3) the dissected form; (4) the grass form; 
(5) the needle form; (6) the roll form; (7) the scale form. In 
many of these, subgroups based upon the structural protection, 
viz., cuticle, hairs, and water cells, may be recognized. 

177. Normal leaf xerophytes. The leaf is normal in size and 
shape, and of the usual dorsiventral character. The necessary 
decrease in transpiration is brought about by structural modi- 
fications, rather than by a reduction in size. Three well-defined 
subtypes may be recognized with respect to the structure used to 




Fig. 49. — A normal leaf xerophyte, Arabis fendleri, in which water loss is 
decreased by a cuticle, and the conversion of the sponge tissue into 
a loose palisade tissue. 

secure protection. These are the cutinized, the lanate, and the 
storage leaf. The cutinized leaf compensates for a low water 
content by thickening the outer wall of the epidermis and render- 
ing it impervious by the addition of cutin. The cuticle thus 
formed sometimes becomes very thick, filling half or more of the 
cell cavity. It is usually thicker upon the upper surface of hori- 
zontal leaves, but is more uniformly developed upon upright or 
oblique ones. The cuticle is often reinforced by a marked develop- 
ment of palisade tissue. Cutinized leaves are usually leathery 
in texture; and in addition, they are often evergreen. Practically 
all xerophytes with smooth leaves of the normal form belong here, 
though many of them have storage cells as well. Good examples 
of this type are found in the bearberry, Arctostaphylus uva-ursi, 
in species of Allionia, Pentstemon, etc. 



158 PLANT PHYSIOLOGY AND ECOLOGY 

178. Storage leaves are distinguished by the water storage cells, 
or tissue developed in the chlorenchym. They usually show a well- 
developed cuticle, with several rows of palisade tissue, and may 
consequently be regarded as a special modification of the cutinized 
leaf. The storage cells maintain a reserve supply of water, which 




Fig. 50. — Cross-section of the two types of storage leaves. The storage 
layers are transverse in Mertensia linearis, and vertical in Grindelia 
squarrosa. 

is slowly yielded to the other cells in time of extreme drouth. 
They differ from the cells of palisade or sponge in size and shape, 
but their origin from these is indicated by the fact that in some 
species chloroplasts are still present, though reduced in number. 
Water cells usually occur in plates or layers, which may be at 
right angles to the surface of the leaf or parallel with it. Xero- 
phytic species of Mertensia, Erigeron, etc., illustrate the more fre- 



ADAPTATION TO WATER 159 

quent arrangement in which the water tissue forms horizontal 
layers, while certain species of Helianthus, Grindelia, Psoralca, etc., 
have this tissue disposed in transverse bundles or rows. 

179. Lanate leaves are those with dense hairy coverings upon 
one or both surfaces. The form of the hairs varies widely in 
different species, from short, often glandular ones to those that 
are repeatedly branched or curved in various ways. An epidermis 
covered with a dense layer of hairs regularly lacks a cuticle. More- 
over, the protection against w r ater loss is so perfect that the chloren- 
chym often assumes the loose structure found in shade leaves. A 
very large number of plants, e.g., Antennaria, Artemisia, Tetra- 
neuris, etc., obtain their protection against drouth by means of 
hairs. In a few cases, the latter are confined to the upper surface, 
but as a rule they are nearly or quite as abundant upon the lower 
surface also. 

Experiment 48. Study of normal leaf xerophytes. Cut cross- 
sections, preferably by means of the microtome, of the three types of 
the normal leaf. Make a careful drawing of a segment across each, 
and in addition outline the entire leaf. Compare the three types crit- 
ically, especially with respect to the various protective devices. This 
comparison is most striking when the three species concerned can be 
found in the same habitat. 

180. Other leaf xerophytes. Species that have lost the normal 
form of the leaf in response to dryness have often found it also 
necessary to employ additional protection. Consequently, they 
may show r a thick cuticle, a hairy covering, or storage tissue. In 
all of the following types, reduction of the leaf surface is the charac- 
teristic feature, though this result may be arrived at in various 
ways, e.g., by thickening the leaf, by lobing, by rolling, etc. 

1. The succulent form. Many succulent leaves are normal in 
shape and size, though they are always thicker than ordinary 
leaves. Usually, however, they are reduced in size, and more or 
less cylindrical in form. The necessary decrease in transpiration 
is secured by reducing the surface, and by storing water uniformly 
throughout the leaf. The latter is usually covered with a waxy 
coating, and often possesses a very thick cuticle. The character 
of the leaf arises from its unusual ability to store w T ater, which 
forms the chief protection of the plant. The stored water is held 
very firmly in opposition to the pull of evaporation. This property 



160 



PLANT PHYSIOLOGY AND ECOLOGY 



is doubtless due to the protoplasm in part, but it arises chiefly 
from a dense or mucilaginous cell-sap. Common examples of leaf 
succulents are the century plant, Agave, the ice-plant, Mesem- 
bryanthemum, and the stone-crop, Sedum, Senecio, etc. 

2. The dissected form. In these the reduction of surface is 
brought about by the division of the leaf blade into narrow linear 




Fig. 51. — A leaf succulent, Sedum stenopetalum. 



or thread-like lobes which are widely separated. The resulting 
decrease in exposed surface is considerable, in some cases exceeding 
nine tenths of the gross outline. The lobes or segments are them- 
selves protected by a hairy covering or a thick cuticle, which is 
often supplemented by many rows of palisade tissue, or by storage 
tissue. Artemisia, Gilia, and Senecio contain xerophytic species 
that are good examples of this type. 

3. The grass form. Xerophytic grasses and sedges have narrow 
filamentous leaves with longitudinal furrows which serve to protect 



ADAPTATION TO WATEB 



1G1 



the stomata. The furrows arc sometimes filled with hairs as an 
additional protection, and the leaves often further reduce their 
surface by rolling up into a thread-like shape. The leaves contain 
also a Large amount of sclerenchyma which renders water loss 
difficult. The elongated awl-shaped Leaves of Juncus and certain 




Fig. 51 



-A xerophytic mat plant with dissected leaves, 
Erigeron pinnatisectus. 



Cyperacece are essentially of the grass type, though they are usually 
not furrowed. 

4. The needle form. This is the typical leaf of pines, spruces, 
and other conifers. It is the result of a sweeping reduction of 
leaf surface made necessary by the persist ance of the leaves during 
winter. The leaves continue to transpire at a time when the 
available water is low on account of freezing, and serious injury 
from drouth is prevented only by greatly reducing the amount of 
surface exposed. The relatively small water loss from the needle 
leaf is further decreased by a thick cuticle, and usually by layers 
of sclerenchyma just below the epidermis. 



162 PLANT PHYSIOLOGY AND ECOLOGY 

5. The roll form. Roll leaves are frequently small and linear. 
Their characteristic form is produced by the rolling in of the 
margin on the under side. This forms an almost completely closed 
chamber for the protection of the stomata, which are regularly 
confined to the lower surface of the leaf. The upper epidermis 
has a thick cuticle, and the lower one is often covered with hairs. 
The roll type is found especially among the genera of the Ericales, 
but it also occurs in a number of other families. 

6. The scale form. The reduction of leaves to scales represents 
the extreme modification of the leaf under xerophytic conditions. 
The next step results in the loss of the leaf and the assumption 
of its functions by the stem. Scale leaves are short and broad, 
leathery in texture, and closely appressed to the stem, as well as 
often overlapping. They are characteristic of many trees and 
shrubs, e.g., Cupressus, Tamarix, Thuja, etc. 

Experiment 49. Study of xerophytic leaves. Species representing the 
above types of leaf xerophytes should be grown in the greenhouse in 
so far as possible. Agave, Sedum, and Bryophyllum serve well for the 
succulent leaf, Artemisia and Gilia for the dissected form, Sporobolus, 
Muhlenbergia, Stipa, and J uncus for the grass form, Erica and Calluna 
for the roll type, while conifers with needle or scale leaves are readily 
found out-of-doors. Sketch a representative leaf of each type, and 
estimate the total surface in square centimeters. Make cross-sections 
and draw a segment from each leaf. 

181. Stem xerophytes. These are characterized by the absence 
of leaves. In some plants the leaves are present at first, but fall early 
in the season. In many cases the leaves are reduced to functionless 
scales or are entirely absent. The functions of the leaf are trans- 
ferred to the stem, which assumes many of the structural modifi- 
cations of the former. The stem or some part of it often becomes 
so changed that it is readily mistaken for a leaf. The following 
kinds of stem xerophytes have been recognized, although not all 
plants of these types are now xerophytes: (1) the phyllode form, 
(2) the virgate form, (3) the rush form, (4) the cladophyll form, 
(5) the flattened form, (6) the thorn form, (7) the succulent form. 

182. Types of stem xerophytes. 1. The phyllode form. The 
petiole is broadened into a leaf-like structure or phyllode. It 
replaces the leaf blade which is entirely lacking. In other cases 
the stem is flattened or winged and takes the place of the whole 
leaf. This type occurs in Acacia, Baccharis, Genista, etc. 



ADAPTATION TO WATER 



163 



J. Thi virgate form. The loaves either fall off early or are 
reduced to functionless scales. The stems are thin, erect, and 
rod-like, and are often greatly branched. They usually possess 
a thick cuticle and much palisade tissue, and the stomata are 
often sunken in longitudinal furrows. This type is characteristic 
of Genista and many of its relatives. It is found also in Ephedra, 
many species of Polygonum, Lygodesmia, etc. 

.;. The ru*h form. In many species of J uncus, Heleocharis, 
Scirpus, and other Cypcraccw the stem is nearly or completely leaf- 




Fig. 53. — A stem succulent, Cactus viviparus. 

less, and it is cylindrical and unbranched. It usually possesses a 
thick cuticle and several rows of dense palisade tissue. 

4. The cladophyll form. In Asparagus the leaves are reduced 
to mere functionless scales, and their work is assumed by the 
small needle-shaped branches. 

5. The flattened form. This is a variation of the preceding 
type. The place of the scale-like leaves is taken by cladophylls, 
which are more or less flattened and leaf-like branches. Ruscus 
is a familiar example of this form. 



164 



PLANT PHYSIOLOGY AND ECOLOGY 



6. The thorn form. This is typical of many spiny desert 
shrubs, in which the leaves are lost very early, or are reduced 
to mere functionless scales. The stems have an extremely thick 
cuticle, and as a rule the stomata are deeply sunken and protected 
by valves. Colletia and Holacantha are good examples of this 
type. 

7. The succulent form. Plants with succulent stems such as 
Euphorbia, Stapelia, and the Cactacece have decreased water 
loss both by the extreme reduction or loss of leaves, and by reduc- 
tion of the stem surface. In addition they guard against exces- 
sive transpiration by means of water-storage tissues containing a 
mucilaginous sap. The cuticle is usually highly developed and 
the stomata sunken. Thorns and spines are also more or less 
characteristic, though they serve only slightly and incidentally 
against water loss. 

Experiment 50. Form and structure of stem xerophytes. Draw in 
careful outline and to scale the stem or shoot of a representative plant 
of each type. Make a drawing of a cross-section of the stem of the 
virgate, or the rush form, and compare with the needle leaf. 

183. Mesophytes. Mesophytic species grow in habitats that 
are neither extremely dry nor wet, and consequently they show 
no striking response to water supply or loss. They possess a 
form or structure that is more or less characteristic by reason of 




Fig. 54. — Cross-section of the leaf of a mesophyte, Pedicularis procera. 



the absence of distinct modifications. As their name indicates, 
mesophytes are middle plants, i.e., they stand midway between 
xerophytes and hydrophytes. For this reason, they pass on the 
one hand into dry land plants, and on the other into water plants. 
More than this, the less intense xerophytes and hydrophytes have 



ADAPTATION TO WATER 165 

often found themselves in conditions that have changed them 
into mesophytes. .Many of the latter have in consequence retained 
characters of leaf, stem, or root which are to be regarded as ances- 
tral rather than as the result of adaptation to the present habitat. 
All of these facts make it unprofitable if not impossible to arrange 
mesophytes under various types. Although they show a large 
variety of forms, these are likewise found in the other two groups. 
Such vegetation forms cannot serve as a basis for separating 
mesophytes into groups based upon the kind or amount of adapta- 
tion to water. 

Mesophytic species fall naturally into the two groups, sun 
plants and shade plants. This is due chiefly to the fact that 
shade in large measure offsets xerophytic conditions, and also 
generally retards the development of hydrophytes. The factor 
concerned here is no longer water content, but light. The char- 
acteristic changes are due to the latter, and sun and shade forms 
are types of adaptation to light. Consequently they are con- 
sidered under the latter. 

Experiment 51. Comparison of mesophyte and xerophyte. Make a 
careful comparison between the form and leaf structure of a mesophytic 
and a xerophytic species of the same genus, e.g., Artemisia, Helianthus, 
Muhlenbergia, Pentstemon, etc. 

184. Hydrophytes. The forms and structures of water plants 
stand out in sharp contrast to those of xerophytes. On the other 
hand, they grade insensibly into mesophytes, and it is impossible 
to draw a sharp line between them. Typical hydrophytes grow 
in water, in soil covered by it, or in saturated soil. With respect 
to their relation to water and air they may be arranged in three 
fairly natural groups, viz., amphibious, floating, and submerged 
plants. In the amphibious form the leaves show the usual 
relation. They grow in the air, while roots and stem are under 
water to a greater or less degree. Floating plants have leaves 
in which the upper surface is in contact with the air, and the 
lower with water. In submerged forms the leaves are usually 
below 7 the surface of the water, i.e., carbon dioxide and oxygen are 
obtained from the water and not from the air. Both surfaces of 
the amphibious leaf and the upper surface of the floating leaf are 
of such a nature as to permit as much transpiration as possible. 
This function is entirely lacking in the leaves of submerged plants. 



166 



PLANT PHYSIOLOGY AND ECOLOGY 



The development of air passages for aeration is great in amphibious 
and floating forms, but they are normally absent from submerged 
plants, in which they persist occasionally as vestiges. Submerged 
plants grow in light that is more or less diffuse, owing to the 
absorption of rays by the water, and their photosynthesis is much 
like that of shade plants, while the other forms are sun plants. 
The fibro vascular system, which is only moderately well developed 
in the amphibious type, is considerably reduced in floating plants, 
and is little more than a remnant in submerged ones. 

185. Amphibious plants. The species of this group are closely 
related to mesophytes: they are the least specialized of water 




Fig. 55. — An amphibious plant, the white marsh-marigold, Caltha lepto- 
sepala. The floating form with long petioles was produced by the 
change of the marsh into a lake. 

plants. As a rule, they grow in saturated soil or in shallow water. 
Owing to their frequent occurrence at the water's edge, many 
amphibious plants have a wide range of adjustment, and may 
grow for a time as mesophytes, or partially submerged. In the 



ADAPTATION TO WATER 167 

majority of cases the leaves are constantly above the water. The 
lower leaves of some species are covered, either normally or by a 
rise in level, and take the form and structure of submerged leaves. 
In Callitriche autumnalis and Hippuris vulgaris, the submerged 
leaves show changes of size and structure, while in the heterophyl- 
lous species, Ranunculus delphinifolius, Proserpinaca palustris, 
Roripa americana, etc., they differ from the aerial leaves in being 
greatly dissected. 

The leaves of amphibious plants, with the exception of those 
just mentioned, are usually large and entire, the stem well developed, 
and the roots numerous and spreading. The epidermis has a thin 
cuticle or none at all, and is destitute of hairs. The stomata are 
numerous and usually more abundant on the upper than on the 
lower surface. The palisade tissue is represented by one or more 
well-developed rows, but this portion of the leaf is regularly thinner 
than the sponge part. The sponge tissue contains large air 
passages, or numerous large air chambers, usually provided with 
thin plates or diaphragms of cells. The stems are often palisaded, 
and are provided with longitudinal air chambers crossed by fre- 
quent diaphragms. 

Experiment 52. Structure of amphibious plants. Draw in outline a 
representative amphibious plant, such as Alisma, Ranunculus, or Sagit- 
taria. Outline an aerial and a submerged leaf of Callitriche or Hippuris, 
and of a heterophyllous species, such as Ranunculus delphinifolius. Draw 
a segment from a cross-section of the aerial and submerged leaves of 
one of the foregoing. 

186. Floating plants. In the form and structure of the upper 
portion, floating leaves are essentially similar to those of am- 
phibious plants. They are usually coated with wax to prevent 
the clogging of the stomata by water. Stomata are found only 
on the upper surface, with the exception of a few cases where they 
persist with loss of function upon the lower side. The palisade 
tissue of the leaf is much less developed than the sponge tissue, 
which is filled with enormous air chambers. The stems or the 
petioles are much elongated, and the aerating system is greatly 
developed, while the supportive tissues, i.e., the fibro vascular 
bundles, are reduced. In the Lemnacece, the leaf and stem are 
represented by a tiny thallus. The roots are in the process of 
disappearing; for example, Spirodela has several, Lemna one, and 
Wolffia none. 



168 



PLANT PHYSIOLOGY AND ECOLOGY 



Experiment 53. Structure of floating plants. Make an outline drawing 
in which Nymphcea and Lemna are contrasted. Draw a cross-section 
of the floating leaf and compare it with a similar section of a Lemna 
thallus. 



187. Submerged plants. 



duced in submerged forms. 




Fig. 56. — Cross-section of a sub- 
merged (3a) and an aerial (36) 
leaf of Callitriche bifida. 



Stem and root are both greatly re- 
This is due to the fact that absorption 
is no longer carried on solely by 
the root, but by the whole stem 
and leaf surface as well. The 
greater density of water as com- 
pared with air also renders sup- 
port less necessary, and the stems 
are unusually long and slender, 
with poorly developed bundles. 
The leaves are greatly reduced 
in size and thickness, and in form 
they are ribbon-like, linear, cylin- 
drical, or finely dissected. These 
changes seem chiefly to serve the purpose of increasing the surface 
for absorption, especially of gases, and for receiving the diffuse 
light rays. Stomata are sometimes present, but they are always 
functionless. The chlorenchym is essentially that of a shade leaf. 
In the few cases where palisade and sponge tissues are present, 
they are doubtless relics of a former structure. The air chambers 
are either much reduced or entirely lacking. When present, they 
probably serve as reservoirs for air obtained from the water. 

Experiment 54. Structure of submerged plants. Make an outline 
drawing of the shoot and root system of Ceratophyllum, Myriophyllum, 
or Philotria. Draw the entire leaf in cross-section. 

Make a detailed comparison by means of a table of the form and 
leaf structure of the various types of hydrophytes. 

188. Bog plants. Many plants of bogs, ponds, banks of streams, 
etc., have the appearance of xerophytes, in spite of the fact that 
they grow in water. Their leaves are more or less reduced, and 
are sometimes lacking. The cuticle is thickened and the palisade 
tissue well developed. The usual explanation of " bog xero- 
phytes " is that they are caused by humic acids in the water, 
which hinder the absorption and aeration of the roots. In other 
words, the available water is thought to be small, though the 



ADAPTATION TO WATER 



169 



total water content is excessive. It has been shown, however, that 
the presence of acids increases the absorption of water. Conse- 
quently, it seems absolutely impossible for small quantities of 
humic acids to produce xerophytes in ponds and bogs. Their 
influence would tend to make water plants even more hydrophytic. 
Moreover, in many ponds and swamps where leafless sedges and 




Fig. 57. — A bog plant, Sagittaria latijolia. The large plant represents the 
normal form, while the small one in the pot is a xerophytic form arti- 
ficially produced to prove that the normal form is not a xerophyte. 



rushes grow, not a trace of acids can be discovered. Furthermore, 
plants which are typical hydrophytes throughout, such as Caltha, 
Ludwigia, Ranunculus, and Sagittaria, are regularly found grow- 
ing alongside of these apparent xerophytes. It is impossible 
for the same habitat to produce both hydrophytes and xerophytes. 
Many of the so-called bog xerophytes possess structures, such as 
air passages, diaphragms, etc., which are peculiar to hydrophytes. 
This is especially true of the form and structure of the root system. 



170 PLANT PHYSIOLOGY AND ECOLOGY 

In consequence, it is incorrect to refer to bog plants as xerophytes. 
In spite of the superficial evidence, they are hydrophytes. 

The presence in bog plants of modifications characteristic of 
xerophytes seems to be explained by the stability of the species 
concerned, i.e., by their ability to adjust themselves to changed 
conditions without undergoing a corresponding change in structure. 
It has recently been shown that certain sun plants undergo no 
material change in structure when grown in the shade. This was 
likewise found to be true of some species growing in two or more 
habitats of very different water content. Hence it is probable 
that the xerophytic features found in some amphibious plants are 
due to the persistence of stable structures. The latter were 
developed when these species were growing in xerophytic situa- 
tions, and not by the hydrophytic habitat in which the plants are 
found at present. The monocotyledons, and especially the grasses, 
sedges, and rushes, are extremely slow in adapting themselves to 
new conditions, i.e., they are very stable. Thus it is readily 
seen how certain ancestral characters may have persisted in spite 
of a striking change of habitat. 

Experiment 55. Study of water-content types. Classify the various 
species found in the field as xerophytes, mesophytes, and hydrophytes, 
and arrange them under the proper types. 



CHAPTER VIII 
ADAPTATION TO LIGHT 

189. The relation of organs to light. Light stimuli call forth 
functional responses which produce changes in form or structure, 
or in both. The latter is the rule, since changes in structure 
really lead to changes in form, as will be seen later. As would be 
expected, the leaf undergoes by far the greatest modification, and 
distinctions between sun and shade plants are based almost wholly 
upon it. The stem shows more or less modification in response 
to light, owing to the fact that it usually contains chloroplasts, 
as well as to the fact that it bears the leaves. The root is with- 
drawn from the action of light and naturally shows only indirect 
effects, such as might result from differences in growth, etc. The 
loss of the leaf or even of the stem or root may occur in very diffuse 
light or in darkness. This is more directly connected, however, 
with the assumption of a parasitic or saprophytic habit. 

190. Influence of the chloroplasts. The clue to the effect of 
light upon the form and structure of leaves is found in the position 
of the chloroplasts. It has already been pointed out that the 
arrangement of the latter varies with the intensity of the light. 
Sunlight causes the chloroplasts to form rows in line with the 
light rays, while diffuse light leads them to take a position at 
right angles to the ray. The three principles upon which the 
structural response of the leaf to light rests are: (1) the number 
of chloroplasts increases with the intensity of the light; (2) in 
diffuse light, i.e., in the shade, chloroplasts arrange themselves in 
such a way as to increase the number exposed to light; (3) in 
sunny habitats, chloroplasts place themselves so that they decrease 
the exposure and the consequent transpiration. 

The chloroplasts lie in close contact with the layer of proto- 
plasm which lines the cell Avail. The latter is itself elastic and 

171 



172 PLANT PHYSIOLOGY AND ECOLOGY 

extensible, and it surrounds a fluid and semi-fluid mass. The 
shape of the cell is in consequence very easily changed. The 
movement of the chloroplasts into lines or rows throughout the 
cell doubtless causes it to elongate in the direction of the rows. 
In the sun the cell thus becomes lengthened in line with the light 
rays, and perpendicularly to the surface of the leaf. It becomes 
a vertical palisade cell, and is termed prolate, since it is drawn out 
in the direction of the poles. In the shade the cell is elongated 
at right angles to the light ray, and parallel to the surface. It 
develops into a horizontal sponge cell, which is flattened contrary 
to the poles, and is hence termed oblate. The form of the sponge 
cell is further modified by the development of air spaces. The 
chloroplasts of some species, especially monocotyledons, do not 
appear to respond to varying light intensities by arranging them- 
selves in rows. In such plants all the cells of the leaf remain 
more or less globose, and there is no distinction into palisade 
and sponge. 

The palisade cell is the normal result of the response of the 
chloroplasts to sunlight. The sponge cell is due to the action 
of diffuse light, or shade, upon the chloroplasts. Palisade cells 
are usually converted into sponge cells in the shade, and sponge 
into palisade in the sun. The latter is illustrated by leaves more 
or less illuminated from below, in which palisade tissue appears 
on both sides. The formation of sponge tissue in diffuse light 
is a characteristic result when plants grow in the shade of others. 
This is equally true of leaves deeply shaded by others of the same 
plant, as is the case in trees and shrubs, and of those which grow 
in the diffuse light of ponds or other bodies of water. The upper 
half of a horizontal leaf shades the lower, producing the typical 
differentiation of the leaf into palisade and sponge. A thick 
covering of hairs shades the palisade tissue, converting it into 
sponge tissue. 

191. Modifications of the chlorenchym. The conversion of the 
chlorenchym into the two tissues, palisade and sponge, is the 
direct result of the unequal illumination of the leaf surfaces. 
This is the normal occurrence in the usual horizontal tj^pe of 
leaf. Exceptions occur only in the monocotyledons already 
noted, in which the leaf tissue consists throughout of sponge-like 
cells. In certain stable species, also, which are now found in 
diffuse light, the palisade tissue has not been developed by the 



AD.UXVTIOX TO LIGHT 



173 





Fig. 58. — Cross-sections of leaves of Allionia linearis, showing the changes 
which the chlorenchym of the sun form (1) undergoes when growing 
naturally in moderate shade (2) with a light value of .012, and in deep 
shade (3) with a light value of .003. 



174 PLANT PHYSIOLOGY AND ECOLOGY 

shade, but represents an ancestral feature that has persisted in 
spite of the change. The difference in the amount of light received 
by the two surfaces is determined by the position of the leaf. 
Leaves that are erect, or nearly so, usually have both sides about 
equally illuminated, and may in consequence be termed isophotic. 
Leaves that stand more or less at right angles to the stem usually 
receive much more light upon the upper than upon the lower 
surface. They really receive two intensities of light, and may 
accordingly be termed diphotic. Certain horizontal or dorsiventral 
leaves, however, absorb nearly or quite as much light on the lower 
side as on the upper. This is true of sun leaves with a dense 
hairy covering, which screens out the greater part of the light 
falling upon the upper surface. It occurs in some degree also in 
xerophytes which grow in light-colored sands and gravels that 
serve to reflect the sun's rays upon the lower side of the leaves. 
In deep shade, moreover, there is little or no difference in the 
strength of the light received by the two surfaces, and shade 
leaves are often isophotic in consequence. Isophotic leaves are 
typical of shaded habitats, but they occur also in the sun. Dipho- 
tic leaves are found chiefly in sunshine, but the diphotic structure 
often persists in shade that is not too dense. It should also be 
noted that the isophotic sun leaf consists chiefly or entirely of 
palisade tissue, while the isophotic shade leaf is similarly composed 
of sponge tissue. Thus, while the direction of the light determines 
whether a leaf shall be isophotic or diphotic, its intensity determines 
the kind of tissue formed. 

192. Sponge tissue. All of the cases just cited make it fairly 
clear that sponge tissue is developed primarily to increase the 
light-absorbing surface. It is found practically without exception 
in all leaves where the light is diffuse, regardless of the cause 
of the latter. The leaves of shrubs and herbs which grow regu- 
larly in forests consist largely or entirely of sponge tissue. The 
interior leaves of the foliage of trees and shrubs contain much 
more sponge than the leaves of the same plant which are exposed 
to the sun. Sun species transferred to the shade usually change 
much or all of their palisade tissue into sponge. This is also 
true of the leaves of amphibious or floating species that become 
submerged, while the leaves of submerged plants consist entirely 
of sponge or sponge-like cells. Further evidence of the develop- 
ment of sponge tissue in consequence of reduced light intensity 



ADAPTATION TO LIGHT 175 

is furnished by diphotic leaves and those with hairy coverings, 
In ordinary diphotic leaves the absorption of sunlight by the 
ehloroplasts of the palisade cells reduces the intensity to such a 
degree that the plastids of the lower half of the leaf are in diffuse 
light. In consequence, the cells that contain them become 
elongated or oblate, and form sponge tissue. In this case the 
latter is just as truly an adaptation to diffuse light as in the pre- 
ceding, where the whole ehlorenchym is in the shade of other 
leaves or plants. A cover of hairs reflects and absorbs the greater 
part of the light which falls upon the leaf, and thereby changes 




Fig. 59. — Leaf of a bog orchid, Gyrostachys stricta, in which the ehlorenchym 
consists wholly of sponge cells. 

the interior into sponge tissue. Consequently, lanate sun leaves 
usually possess ehlorenchym essentially like that of shade leaves. 

From the preceding it seems clear that sponge tissue serves 
primarily to increase the light-absorbing or chlorophyll surface 
under all conditions that make such an increase beneficial. In 
addition, it is intimately connected with aeration, largely owing 
to the fact that it is in contact with the low T er epidermis, which 
usually contains the larger number of stomata. Indeed, the 
spongy nature of this tissue is due to the presence of air spaces, 
which are the means of carrying on effective aeration. This point 
is nicely brought out by submerged leaves, in which the form of 
the cells and the arrangement of the plastids are those of sponge 
tissue. The usual air spaces are altogether lacking, however, 
since the air is now obtained in solution in the water. Finally, 
the abundance and size of the air spaces in the sponge makes the 
latter subject in some degree to the modifying influence of tran- 



176 



PLANT PHYSIOLOGY AND ECOLOGY 



spiration. The fact that light is the most potent factor in pro- 
ducing sponge tissue is proved by the uniform occurrence of the 
latter in the lower half of the leaf, when the stomata are more 
numerous upon the upper surface. 

193. Palisade tissue. The formation of palisade tissue is 
brought about in response to sunlight or to low water content. In 
very many cases the two act together and produce a striking 




Fig. 60. — Cross-section of leaves of the sun (1) and shade (2) form of a 
sunflower, Helianthus pumilus, in which the chlorenchym consists 
entirely of palisade tissue. 



modification. In amphibious and floating plants, and in many 
mesophytes where the water supply is adequate, the palisade is 
due to the action of light. On the other hand, it is possible to 
increase the number of rows of palisade cells in the leaves of most 
sun plants by growing them in drier conditions. As a consequence, 
it is a difficult task to decide which factor is the more important 
in producing this tissue. When it is recalled that the light energy 
is almost entirely used in causing evaporation from the plastids, 



ADAPTATION TO LIGHT 177 

it is evident that palisade tissue is primarily, if not wholly, a 
protection againsl water loss. Since the latter is due to light 

or to water, palisade may be developed chiefly by the one or the 
other according to conditions. 

Protection againsl water loss in consequence of sunlight is 
brought about by the arrangement of the chloroplasts in vertical 
rows so that they screen each other. Palisade cells greatly 
decrease the transpiration due to low humidity by being so closely 
placed that the outward movement of the moist air is hindered. 
The air passages between them are usually reduced to the narrowest 
of chambers, and often to mere lines. The truth of these state- 
ments is not affected by the fact that leaves with much palisade 
tissue, i.e., sun leaves, sometimes transpire twice as much as 
shade leaves of the same size. This is explained by the much 
greater activity of chloroplasts in the sunlight. As a result, sun 
leaves require a larger supply of carbon dioxide, and the number 
of stomata is correspondingly increased, often being doubled. 
In consequence, the loss of water through the stomata is neces- 
sarily increased. The small size of the air passages in palisade 
tissue would seem to prevent the rapid movement of carbon 
dioxide to the chloroplasts. This is apparently compensated 
by the fact that the greater demand for this gas causes it to move 
most rapidly toward those points where it is being used in the 
largest quantities. 

194. Changes of the epidermis. The appearance of the chloro- 
plasts in the epidermal cells of plants growing in diffuse light is the 
only change directly traceable to light. Chloroplasts are regularly 
present in the epidermal cells of w r oodland ferns and of submerged 
plants. They are also found in those shade forms of sun species 
in which the outer wall of the epidermis has become thin enough 
to admit carbon dioxide. The absence or slight development of 
hairs in shade plants is an advantage, because it prevents the further 
weakening of the already diffuse light. The value of arched epi- 
dermal cells and of epidermal papillae in controlling the absorption 
of light by shade plants seems to be slight. The factor that has 
called forth these modifications and the primary purpose they 
serve must still be regarded as unsettled. The increased size of 
the epidermal cells in many shade forms seems to be for the purpose 
of increasing translocation and water loss, and to bear no direct 
relation to lisjht. The extreme size of these cells in certain mono- 



178 PLANT PHYSIOLOGY AND ECOLOGY 

cotyledons growing at the edge of shaded brooks is probably a 
contrivance to increase water loss. 

The number of stomata for an equal area of the epidermis is 
greater in sun leaves than in shade leaves. This is generally true 
of all sun and shade plants, but it is most clearly shown by the 
different habitat forms, or ecads, of the same species. The sun 
leaf of Allionia linearis has 180 stomata, and the shade leaf 90 
stomata per square millimeter. In Scutellaria brittonii, the 100 
stomata per sq. mm. of the sun leaf are reduced to 40 in the shade 
leaf. However, in a stable species such as Erigeron speciosus, 
the number of stomata remains unchanged in those plants that 
have moved into the shade. The presence of the larger number of 
stomata in the sun plant, which is exposed to the greater water 
loss, has already been explained. 

195. The form of leaves. The form of the leaf is largely deter- 
mined by the action of light upon the chloroplasts and the conse- 
quent change in the form of the cells that contain them. Owing 
to the direction in which they elongate, sponge cells tend to produce 
an extension of the leaf at right angles to the light rays. On the 
other hand, palisade cells extend the leaf in line with the falling 
rays. In consequence, leaves which contain an excess of sponge 
tissue are relatively broader, while those in which palisade is 
preponderant are relatively thicker. Since plants economize 
material and energy in so far as possible, the broadened leaf tends 
to be thin, and the thickened leaf to be narrow. In accordance, 
shade leaves, i.e., those that consist largely or wholly of sponge 
tissue, are broader and thinner, and often larger, than sun leaves 
of the same species. Sun leaves, on the contrary, are thicker, 
narrower, and often smaller than shade leaves. What is true of 
the sun and shade forms of the same species holds for sun and 
shade plants generally. 

196. Changes of outline, size, and thickness. The outline of 
shade leaves is more nearly entire than that of those in the sun. 
This is easily proved by comparing the sun and shade forms of 
a species with lobed or divided leaves, though the rule is not with- 
out exceptions. In Fig. 21 the outline of the shade form is more 
entire in Bursa and Thalictrum, but less entire in Machairanthera. 
Leaf prints of this kind serve more satisfactorily to illustrate the 
increase in size and decrease in thickness produced by the increase 
of surface in the shade leaf. In all such comparisons, however, 



ADAPTATION TO LKJIIT 



179 



the relative size and vigor of the sun and shade plants must be 
taken into account. The relation between surface and thickness 
is shown by the following species, in all of which the leaf is larger 
in the shade than in the sun. In AUionia linearis, the thickness 
of the shade leaf is one fourth that of the sun leaf, i.e., the ratio is 
3: 12. and mCajmoidesaureum 6:12. The ratio in Thalictrum sparsi- 
florum is 9:12, and in Machceranthera aspcra 11:12. The ratio of 
thickness of the shade and sun forms of Bursa bursa-pastoris is 




Fig. 61. — Sun and shade lorm of Senecio taraxacoides. The leaves are 
larger, thinner, and smoother in the shade plant, and usually more 
entire. 

14:12, but the greater thickness of the shade leaf is explained by 
the fact that this plant is ten times larger than the sun form. 
Upon the basis of size, the thickness of the shade leaf is scarcely 
one ninth that of the sun leaf. Some species show no change in 
thickness, and but little in size or outline. This is doubtless to be 
explained by the fact that the form is so fixed in the sun plant that 
the decrease in light intensity has little or no effect upon it. 

197. The form of stems. Shade ecads are regularly taller and 
often more branched than the corresponding sun form. In general, 
this statement is true of all shade plants as compared with sun 



180 



PLANT PHYSIOLOGY AND ECOLOGY 



plants. The effect of diffuse light in causing stems to stretch 
upward has been known for a long time, but the way in which 
it is brought about is still unexplained. It is often stated that 
the stem elongates to obtain more light, but the only basis for 
this is that the stem grows in the direction from which the light 
comes. This elongation is much more marked in the case of 
plants grown in the dark, in which the height of the plant has 
no bearing upon the amount of light that it can obtain. It has 
been shown that the stretching of the stem is due to the excessive 




Fig. 62. — Modifications of the stem in several forms of Androsace diffusa. 
The rosette is an alpine plant growing at 3800 m.; the middle plant 
grows in the open gravel at 2600 m. and the branched plant in the shade 
at 2600 m. 



elongation of the parenchyma cells, but the cause of the latter 
is in doubt. It is generally thought to be the absence of the usual 
action of sunlight, which is assumed to be a retarding of growth 
in sun plants. The evidence in favor of such a view is far from 
conclusive. It seems probable that the elongation of the paren- 
chyma cells takes place under conditions which greatly promote 
the mechanical stretching of the cell wall, but prevent the normal 
growth of the latter by intussusception. The fact that photo- 
synthesis, and hence the amount of constructive material, is 
greatly reduced in shade plants favors such an explanation. What- 



ADAPTATION TO LIGHT 



181 



ever the cause may be, it is evident that the elongation of the 
stem is an advantage to plants that grow in diffuse light. This 
holds for submerged as well as for shade plants. Upon a stem 
with elongated internodes, the leaves interfere less with the illu- 
mination of those below them. This is also true of the branches, 
which serve further to carry the leaves away from the stem and 
from each other in such a way that the plant obtains the greatest 
possible exposure of its leaf surface. 




Fig. 63. 



-Shade and sun plants of Gaura parvi flora, the former produced in 
a shade tent. 



Experiment 56. The production of adaptations to light. Construct a 
series of shade tents 1-2 meters square and high. This may be done 
in the greenhouse or out-of-doors by using wooden strips of 3-5 cm. to 
make a framework of the size desired. The latter is divided into three 
parts, and each is covered with cloth of the proper texture to give the 
light intensity sought. It is desirable to have a series of tents with light 
intensities 0.1, 0.05, and 0.01 of the normal sunshine out of doors. These 
values may be approximated by cheese-cloth, muslin, and duck, and after 
a few trials may be secured almost exactly. Each tent is covered by 
tacking the proper cloth upon it. The interior walls are usually made 
of the cloth belonging respectively to the second and the third tent. 
On one side the cloth is not tacked, but is arranged to button closely 



182 PLANT PHYSIOLOGY AND ECOLOGY 

on the framework in order to serve as an opening. Ordinarily it is not 
necessary to ventilate each tent, but, if the humidity within becomes 
too high, this is effected by means of hoods at the top or sides. 

Practically all flowering plants will show some adaptation to the 
different light intensities of the shade tent. Helianthus, Allionia, 
and Taraxacum have been found especially suited for this work, though 
a number of others, Bursa, Galium, Onagra, etc., are equally good. 
Helianthus annuus (wild form) illustrates fairly well the behavior of a 
stable species, and Allionia linearis or A. nyctaginea that of a plastic 
one. Taraxacum taraxacum shows the effect of shade upon divided 
leaves and the form of the rosette. Both Bursa and Onagra are espe- 
cially good to show the stretching of the petioles and internodes. It 
is best to start the seedlings under normal conditions and then to place 
4-6 of each species in each tent as soon as they begin vigorous growth. 
A set should likewise be left in normal light to serve as checks. 

During the growth of the plants make occasional readings of light 
intensity for the three tents, and make one determination of the starch 
content of a representative leaf of each species for each tent. Follow 
with care the differences in the growth and behavior of each species 
in the three tents and in the sunlight. When the plants are well grown, 
make an outline drawing of the leafy plant of each species for the four 
conditions and a similar outline of a representative leaf of each, drawing 
all stems as well as leaves to the same scale. Select a representative 
leaf from each form, and make a leaf print for the four forms of each 
species. Kill a similar set of leaves from each species and make a 
study of the structural modifications as shown by microtome sections. 
A similar study of modifications in the number of stomata may well 
be made by stripping the epidermis from the fresh leaves. Prepare a 
concise account of the adaptation of the species concerned to different 
light intensities. 

198. Types of leaves as determined by light. Isophotic 
leaves are equally illuminated on both surfaces, or nearly so, and 
possess a more or less uniform chlorenchym. Diphotic leaves 
are unequally illuminated, and show a division into palisade and 
sponge tissue. The ordinary horizontal or dorsiventral leaves 
are usually diphotic. Leaves of this type contain both palisade 
and sponge, though the relative importance of the two varies 
considerably in different species. Diphotic leaves are character- 
istic of sunny swamps, meadows, prairies, etc., and are frequent 
in xerophytic habitats. Floating leaves, in which the light is 
almost completely cut off from the lower surface, are also diphotic. 
This type of structure is often found in the leaves of shady 



ADAPTATION TO LIGHT 



183 



forests, in which it is a relic of the Btructure of the ancestral sun 
leaf. 

[sophotic loaves fall into three types, based upon the intensity 
of the light. The palisade leaf, or staurophi/ll, is a sun leaf in 
which the chlorenchym consists wholly of rows of palisade cells. 
It is produced by nearly equal illumination of both surfaces, due to 




Fig. 6-4. — Isophotic leaf (1) of an oak, Quercus novimexicana. The shade 
leaf (2) is of the same type, though one row of palisade tissue is lost 
and the leaf is thinner. 

its upright position or to reflection from a light-colored soil. In a 
special form of this type, the diplophyll or double leaf, the intense 
light does not penetrate to the middle of the leaf. In consequence, 
the upper and lower palisade areas are separated by a central 
sponge-like tissue, w r hich is used for the storage of water. The 
sponge leaf, or spongophyll, includes all shade leaves, except those 
in which some palisade persists from the ancestral sun form, and 
practically all submerged leaves. Its chlorenchym consists of 
sponge-cells alone. Certain monocotyledons, wTiich grow in the 
sun but lack palisade, may also be referred to this type for the 
present. 

199. Sun plants and shade plants. Sun plants are also termed 
heliophytes and shade plants sciophytes. The former comprise 
practically all xerophytes, prairie and meadow mesophytes, and 
amphibious and floating hydrophytes. Shade plants include 
the mesophytes of thickets and forests and submerged hydro- 
phytes.- The differences in the form of stem and leaf shown by 
these tw r o types have already been discussed. The greater number 



184 



PLANT PHYSIOLOGY AND ECOLOGY 



of sun plants are diphotic, as represented by Pedicularis procera 
(Fig. 54). Plants with isophotic leaves are found frequently 
in xerophytic places, though erect leaves of this type occur in 




Fig. 65. — Isophotic leaf (1) of Bidens bigelovii, which in the shade form (2) 
becomes practically a spongophyll by the reduction of the palisade 
layer. 

most sunny habitats. The staurophyll, in which protection is 
due to the extreme development of palisade tissue, is illustrated 
by Allionia linearis (Fig. 58) and Bahia dissecta (Fig. 47). The 
diplophyll, which is characterized by a central band of sponge 
tissue or storage cells, is found in Mertensia linearis (Fig. 50). 
The spongophyll is frequent among plants of deep shade, but, as 
the leaf sections of Allionia (Fig. 58) and Quercus (Fig. 64) show, 
the diphotic leaf is equally common among shade plants. The 
form of sun spongophyll found in certain monocotyledons is 
shown in Gyrostachys stricta (Fig. 59). 

Experiment 57. Sun and shade forms in nature. Make a list of all 
the genera of the local flora in which sun and shade species occur, and 
make a general comparison of the latter. 

Make a thorough search for sun and shade forms of the same species. 
Such species, called polydemics, are especially apt to be found near 
the border of grassland and woodland, where species may wander easily 
into either habitat. Make a study of the sun and shade forms of a 
polydemic by means of outlines and leaf prints. 



CHAPTER IX 
THE ORIGIN OF NEW FORMS 

200. The law of evolution. Evolution is the production of 
a new plant form out of an existing one. It is commonly spoken 
of as the origin of species, but this expression is far from exact. 
All plant groups, forms, varieties, species, genera, etc., regardless 
of their rank, are products of evolution. The term species, 
moreover, has become so vague that it no longer has definite 
meaning from the standpoint of evolution. Properly speaking, 
the latter is the origin of all new forms. The existence of such 
a universal process is now beyond question. The exact ways 
in which new forms arise and the factors which control their 
origin are still imperfectly known. Evolution has scarcely entered 
the experimental stage. There has been a surplus of works and 
papers upon this subject, but with few exceptions they have added 
nothing to our real knowledge of it. 

201. Stability and plasticity. Evolution is the process in which 
organisms are changed by the immediate or remote action of their 
environment. The exact connection between many changes in 
plants, for example, and their habitat has not yet been made, 
owing to the extremely small amount of experimental study. 
The feeling that all changes can be traced sooner or later to the 
factors of the habitat arises from the belief that every form has 
descended from the primitive protoplasm, solely in consequence 
of changes w r rought in the latter by the habitat. The most con- 
vincing evidence in favor of this belief, which naturally can never 
be proved or disproved, is found in the fact that every form used 
for experiment can be changed in response to changing factors. 
Consequently we are justified in assuming as a working hypothesis 
that all plants can be changed by means of the habitat. To 

185 



186 PLANT PHYSIOLOGY AND ECOLOGY 

obtain the proof of this and of the manner in which it occurs is 
the task of experimental evolution. 

Forms which grow for a long time in the same habitat seem to 
fix more and more those functions and structures which are the 
responses to it. They may be said to acquire habits which become 
more fixed the longer the causes act. Of this tendency to fix 
characters, there is as yet no definite and complete experimental 
proof. Sufficient evidence to warrant its use as a working hypoth- 
esis is found in the behavior of plants in nature and in experiment. 
Some forms or species show little or no change when grown in 
greatly changed conditions, while others respond to slight differ- 
ences in habitat. The former are said to be stable, the latter are 
termed plastic. The great majority of plants are neither extremely 
stable nor extremely plastic. Some are more stable, others less 
so. Some clue to this may be obtained from observations made 
in the field, but the final test of a plant's stability must be made 
by growing it under changed conditions. 

The amount of stability shown by a plant determines to what 
degree evolution or change is possible for it. In other words, the 
form and structure inherited by the plant from its ancestors not 
only constitutes the material acted upon by evolution, but it also 
determines how far the change may go. This is the historical or 
ancestral factor in evolution. The change which the plant under- 
goes is the result of a change in the habitat, which is the physical 
factor in evolution. The amount of change or modification in 
the plant depends upon the intensity of the change in habitat, i.e., 
upon the stimulus as well as upon the degree of stability. Evolu- 
tion is thus seen to be the result of two opposite tendencies, sta- 
bility and change. When the former predominates, evolution is 
either very slow or very slight. In extreme cases it may be 
impossible. When the second tendency is stronger, evolution is 
rapid, and a new form appears in response to each change in habitat. 
In consequence, while we shall see that evolution may be brought 
about in several ways, every instance of it is at bottom a question 
of the relation between the antagonistic tendencies, stability and 
change. 

202. Constant and inconstant forms. New forms that arise by 
evolution produce offspring similar to themselves as long as the 
offspring remain in the home. If the offspring invade new and 
different habitats, they may retain the characters of the parent 



THE ORIGIN OF NEW FORMS I s ' 

or they may he modified under proper conditions, returning to the 
form from which the parent sprang. Forms which remain true 

to the parent type under different conditions are said to be 
constant, while those that revert to the type preceding the parent 
are termed inconstant. 

For a great many years species were supposed to be constant, 
but varieties and forms inconstant. Constancy was rarely made 
a matter of experiment, however, and was practically never used 
in connection with the naming of new species. In consequence, 
many species of the manuals are not constant, and many varieties 
and forms are. The distinction between these three disappears 
accordingly. It has slight meaning to distinguish one new form 
as a species and another as a variety. The use of either term in 
any exact scientific manner is difficult until the present so-called 
species are thoroughly examined experimentally, and new criteria 
are established as a result. The common usage of descriptive 
botany is to term new forms species, regardless of the way in 
which they originate or the amount of difference they show. It 
is necessary to distinguish different kinds of species, or to make 
a definite distinction between species and other forms. The 
basis for either procedure must be experiment and not observation 
merely, though the latter is often an aid. 

Constancy is in nowise a test of evolution, though it plays an 
important part in the arrangement of the new forms that arise. 
The way in which' new forms originate determines whether they 
shall be constant or not, i.e., constancy is itself a result of evolu- 
tion, rather than a factor in it. Moreover, it seems very probable 
that constancy is directly influenced by the habitat. A shade 
form that has sprung from a sun plant usually reverts at once 
to the original form if the seeds of the first generation are grown 
in the sun. This reversion seems to take place more slowly after 
a number of generations, and it is probable that it would be 
slower or more incomplete after a hundred or a thousand genera- 
tions. 

203. Origin by descent before Darwin. Before the appearance 
of Darwin's " Origin of Species" in 1859, it was commonly be- 
lieved that genera and species were the result of special creative 
acts. Varieties, on the contrary, were supposed to arise from 
species through the influence of external conditions. This was 
the view held by Linnaeus, whose authority was such as to cause 



188 PLANT PHYSIOLOGY AND ECOLOGY 

its almost universal acceptance. Bacon, 1 in his " Natural His- 
tory" (1658), seems to have been the first to state that one kind 
of plant may change into another through transmutation. Among 
the causes of the latter he mentions drouth especially, further 
pointing out that it does not act if the earth be moist. His 
greatest achievement, however, was in anticipating in definite 
though crude fashion the methods of experimental evolution, as 
shown by his six rules for making one plant change into another. 
One of these was "to take marsh herbs and plant them upon the 
tops of hills and champaigns, and such plants as require much 
moisture upon sandy and very dry grounds." Another was "to 
make plants grow out of the sunshine, since this was a great change 
in condition, and might bring about a change in the seed." Bacon's 
ideas, though many were necessarily crude and incorrect, indicate 
clearly that he had observed the origin of new forms by adaptation 
to the habitat, and believed that such forms could be produced 
experimentally. 

The first writer whose views on evolution attracted serious atten- 
tion was Lamarck. 2 His ideas were first advanced in 1801, and 
further enlarged and revised in 1809 and 1815. He was the first 
to point out clearly that all species have descended from other 
species. Lamarck believed that new forms arose in three ways: 
by the direct action of the habitat, in consequence of the use and 
disuse of parts, and through crossing of existing forms. He held, 
moreover, that evolution takes place in conformity with the law 
of progressive development, and, to explain the universal presence 
of simple forms, he assumed that these are arising constantly out 
of non-living material. Within recent years, many of Lamarck's 
views have been widely adopted by biologists, who are accordingly 
known as Neo-Lamarckians. Saint-Hilaire 3 in 1828 reached the 
conclusion that the species of to-day have descended from earlier 
ones through the modification of the latter. He regarded the 
habitat as the cause of change, as is clearly shown by his state- 
ment that "specific characters remain fixed for each species as 
long as the latter grows under the same conditions: they are modi- 
fied in case the habitat undergoes a change." 

204. Darwin and the Origin of Species. The preceding account 

1 Bacon, Francis. Sylva Sylvarum or a Natural History, 110, 1658. 

2 Philosophic Zoologique, 1809. 

3 Sur le Principe de 1'Unite de Composition Organique, 1828. 



THE ORIGIN OF NEW FORMS 189 

makes it evident that Darwin was not the discoverer of the law 

of evolution, contrary to what is often assumed. In an historical 
sketch of the progress of opinion on the origin of species previous 
to the appearance of his own book, Darwin himself summarizes 
the views of twenty biologists who gave entire or partial support 
to the idea of evolution. The action of natural selection as a 
factor in evolution, the discovery of which has been commonly 
attributed to Darwin, was first suggested by Wells in 1813. Matthew 
in 1831 and Naudin in 1852 held the same view of the importance 
of natural selection as that advanced independently by Darwin 
and Wallace in 1859. Nevertheless, while it is incorrect to ascribe 
the discovery of evolution and natural selection to Darwin, he 
must receive the fullest credit for bringing about the final accept- 
ance of origin by descent. His tw r enty years of painstaking study 
of evolution left no doubt of its being a universal process, even 
though he was unable to prove the exact way in which it acts. 

Danvin recognized that new forms arose through the direct 
action of the habitat and through the production of sports. He 
considered that the action of the habitat led to definite or in- 
definite variation, while it was impossible to connect the origin 
of sports with external causes. Definite variation occurs w T hen 
all or nearly all individuals respond in the same way, while in- 
definite variation, called also fluctuating variability, takes place 
when the individuals are slightly modified in all directions. While 
Darwin believed that definite variation, i.e., adaptation, as well 
as sports, i.e., mutation, occasionally produced new r forms, he 
held that species ordinarily arise in consequence of indefinite 
variation. The minute variations of individuals were assumed 
to be preserved and accumulated through natural selection, or 
the survival of the fittest. The latter, moreover, was supposed to 
be due entirely to the competition between individuals and not 
to the direct action of the physical factors of the habitat. Dar- 
win's conclusions were based chiefly upon the study of domesticated 
plants and animals, and upon observation instead of upon ex- 
periment. These two facts serve to explain why he found it 
possible to put the greatest emphasis upon origin by variation 
and natural selection, which is, of the three methods recognized 
by him, the only one not experimentally proven. 

205. Evolution after Darwin. As frequently happens after the 
appearance of a great work, the "Origin of Species" ushered in a 



190 PLANT PHYSIOLOGY AND ECOLOGY 

period of theoretical discussion of slight value. This was especially 
unfortunate, since it almost completely obscured the fact that the 
value of Darwin's hypotheses could be tested by experiment alone. 
Hence in this period it is necessary for us to consider only the 
conclusions of Henslow and De Vries. The former obtained much 
new material from a field neglected by Darwin, viz., the origin 
of adaptations in nature, while the latter by means of careful 
experiments placed beyond question the origin of new forms 
through mutation. Henslow 1 believes that Darwin was wrong in 
assuming that "indefinite variability is a much more common result 
of changed conditions than definite variability." His own opinion 
is that "in nature variations are always definite and not excep- 
tionally so : the consequence is that ' all or nearly all the individuals 
become modified in the same way' (and) the result is that a new 
variety and thence a new species 'would be produced without 
the aid of natural selection.'" His final conclusion is that "the 
origin of species is due to the joint action alone of the two great 
factors of evolution — variability and environment — without the 
aid of natural selection." 

De Vries 2 states that while "the current belief assumes that 
species are slowly changed into new types, in contradiction to this 
conception, the theory of mutation assumes that new species and 
varieties are produced from existing ones by sudden leaps." His 
conclusions are based chiefly upon the experimental study of an 
evening primrose, (Enothera lamarckiana. He found that out of 
a hundred or more species in nature, this was the only one that 
suddenly produced new forms, i.e., mutations. From it he ob- 
tained in the field and in garden cultures twelve mutations or 
" new elementary species." These arose suddenly from the parent 
stock, without any connection with the habitat, and came true 
from seed. A careful examination of De Vries' results leaves no 
doubt that mutation is proved to be one of the methods by which 
new forms originate. That it is the only method of origin is 
certainly not true. Moreover, it is perfectly evident that De Vries' 
experiments upon (Enothera are quite inadequate to prove it the 
chief method, as he would have us think. It is difficult, more- 

1 Henslow, George. The Origin of Plant Structures by Self-adaptation 
to the Environment, IX, 1895. 

2 De Vries, Hugo. Die Mutationstheorie, 1901; Species and Varieties, 
their Origin by Mutation. 1905. 



THE ORIGIN OF NEW FORMS 



191 



over, to accept his statement to the effect that his work is "in 
full accord with the principles laid down by Darwin." The latter 
held that new forms arise regularly from indefinite variations; 
I)e Vries derives them from mutations; Darwin regarded natural 
selection as a necessary agent in originating new forms by varia- 
tion, while to De Vries it is merely a process that acts after origin 
is complete. 

206. Fundamental methods of evolution. From the foregoing 
it is evident that new forms probably originate in one of three 




Fig. 66. — Solidago oreophila and its alpine form, Solidago decumbens. The 
latter is due to dwarfing, arising from low temperature and from the 
increased water loss caused by decreased pressure at high altitudes. 

different w r ays, i.e., by indefinite variability or variation, by 
definite variation or adaptation, or by mutation. New forms 
arise also by crossing or hybridization, which has usually not 
been accounted a method of evolution. It is interesting to note 



192 PLANT PHYSIOLOGY AND ECOLOGY 

that Bacon, Lamarck, and Saint-Hilaire held that evolution 
proceeded chiefly from adaptation. Darwin, while recognizing 
the occurrence of both adaptation and mutation, was led to think 
that variation was the common method of origin. Henslow and 
many other Neo-Lamarckians held that adaptation is the uni- 
versal method, refusing to accept variation and neglecting mutation. 
De Vries, on the other hand, eliminates adaptation as unable to 
produce constant forms, points out that variation has never 
been proved to originate new forms, and consequently regards 
mutation as the universal process. In these extreme views there 
is both truth and error. Adaptation, mutation, and hybridation 
have been proved by experiment to be able to produce new and 
distinct forms. While similar proof is lacking in the case of 
variation, it seems probable that it is because no careful experi- 
ments have yet been made in regard to it. 

207. Origin by adaptation. New forms may originate in 
nature by adaptation to the physical factors in consequence of 
invasion into a different habitat, or of a marked change in the 
same habitat. It is equally clear that they may be produced 
artificially. In either case the chances that a new form will arise 
depend almost wholly upon the stability of the original form. 
An extremely stable form remains essentially the same in all 
habitats in which it can grow. A very plastic one gives rise to 
a new form whenever it enters a new habitat or has its own changed. 
Origin by adaptation occurs universally in the case of plastic 
species whenever they are placed under different physical factors. 
The factors that control origin of this sort are the direct ones, 
water and light. The ways in which plants respond to them 
by changes in form and structure have already been discussed 
in the two chapters preceding. A new form arising from adapta- 
tion is called an ecad. For example, the seeds of a plastic sun 
plant that has entered a forest develop into a shade ecad, while 
a prairie species carried into a bog may give rise to a water ecad. 
A species able to invade successfully two or more different habitats 
will produce a corresponding number of ecads, provided it is not 
too stable. 

Since plastic species are the only ones that give rise readily to 
new forms, it is to be expected that such forms will respond with 
similar readiness to new conditions. A shade ecad can be made 
to return to the parent form by transferring its seeds to the sun. 



THE ORIGIN OF NEW FORMS 



193 



That all ecads will do so upon bring returned to the original habitat 

can not be told until each one has been placed under experiment. 
This appears to be probable, but preliminary results indicate 
that the longer an ecad remains in the habitat that produced it 
the more difficult a change becomes. This is in accord with the 
general opinion that stability is merely fixed habit. Hence the 
Longer a plant is in the habit of carrying on its functions or pro- 




Fig. 67. — The sun form of the mountain skullcap, Scutellaria brittonii, and 
the new form which arises from it by adaptation to deep shade. 

ducing its structures in a certain way the more stable it becomes. 
In nature ecads are frequent, occurring more or less commonly 
wherever distinct formations touch each other. While they may 
wander back at any time to the original home and revert, they 
ordinarily persist for years, often doubtless for centuries, as distinct 
new forms. In amount of difference they are as distinct as many 
new species and have often been described as such. Whether they 



194 



PLANT PHYSIOLOGY AND ECOLOGY 



are species or not depends entirely upon the meaning given to 
this term. For the present it is clearer to use the term ecad 
for all new forms arising by adaptation. 

Adaptation is closely related to variation. Both are probably 
equally due to response to the habitat, but they differ in amount 
and direction of responce. The latter is definite in the one case, 
indefinite in the other. Origin by adaptation, like mutation, 



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Fig. 68. — A floating form of a crowfoot, Ranunculus sceleratus, produced by 
artificial adaptation. 

takes place quickly, usually in a single generation. Variation 
must work slowly through many years, in consequence of the 
heaping up of minute differences. In this process natural selec- 
tion is the essential factor; in adaptation while present it is much 
less evident. Moreover, while origin by variation is still a doubtful 
factor in evolution, it is probable that origin by adaptation is the 
most frequent method found among plants. 



Till. ORIGIN OF NEW FORMS 



19" 



208. Origin by variation. Variation IS here used only in the 
sense of indefinite variability, i.e.. very slighl differences of any 
OT all parts in any direction. A careful scrutiny of many indi- 
viduals oi the same species quickly reveals the fact thai do two 
are exactly alike. There are many slight differences of size, color, 
form, surface, etc., some found in one plant, some in another, and 






Fig. 69. — Rosettes of the fireweed, Chamcenerium angustifolium , showing 
striking variations in the shape of the leaf and the length of the petiole. 

so forth. Danvin pointed out that in the competition between 
the various individuals some of these slight variations are favorable 
to success, others unfavorable. The individuals with favorable 
variations obtain a certain small advantage over the others. In 
consequence they grow larger and stronger, are more attractive 
to insects, etc., and as a result produce more or better seeds. 
Accordingly, in the second generation, the favorable variations are 



196 PLANT PHYSIOLOGY AND ECOLOGY 

represented by a larger number of individuals, the unfavorable ones 
by fewer. This process by which competition, the factors of the 
habitat, or the two acting together, pick out certain plants to the 
disadvantage of others is called natural selection. Essentially 
the same thing occurs in artificial selection, when the florist selects 
for propagation plants that have a desired feature, and neglects 
or destroys the remainder. Darwin felt, moreover, that favorable 
variations tended to grow more and more marked with each 
succeeding generation. As a consequence, the individuals showing 
these would become more and more numerous and distinctive, 
while those with other modifications would decrease and finally 
disappear. After many years, probably after several centuries, 
a form sufficiently distinct to be called a variety or species would 
be produced. Forms arising in such a manner are termed variants. 

The critical point in the theory of origin by indefinite variation 
is the action of natural selection in preserving and accumulating 
minute differences. The presence of selection in nature is uni- 
versally recognized. The tendency for a habit to become fixed 
is generally conceded. Yet it must be stated that there is no 
experimental proof that natural selection acts in the way assumed 
by Darwin. Until such evidence is obtained from careful experi- 
ments, it must remain doubtful whether new forms can be produced 
by variation and natural selection. 

209. Origin by mutation. Mutation takes place when one 
or more individuals of a form show a sudden and more or less 
marked departure from it in one or more features. Differences 
of this sort are extremely rare in nature, at least in comparison 
with adaptations and variations. Sports are more frequently seen 
among cultivated plants, probably owing to the intensive action of 
cultivation. In nature one may expect to find a white-flowered 
sport of any species with red, blue, or purple flowers, but the most 
minute search reveals few other mutations. When the latter 
occur they are ordinarily represented by very few individuals. 
A new form arising by mutation is termed a mutant. 

A mutant can rarely be traced to the direct action of the 
habitat. It is probable, however, that it is the result of delayed 
or latent response to some change in factor, or of a series of 
responses set up in the plant by a factor. A species may show 
mutation in any direction, just as is true of variation. In fact, 
mutation seems to be merely the appearance of variations 



THE ORIGIN OF NEW FORMS 



197 



accumulated within the plant. The difference between a variant 
and a mutant is apparently one of degree and not of kind. In 
the one case, however, natural selection plays a necessary part 
in the process, while with mutants it merely determines whether 
they shall persist. The features of a mutant may be unfavorable 
as well as favorable. In addition, a mutant crosses readily with 
the parent form. In consequence, it runs many chances of disap- 
pearing or of being merged with the original form. When the 




Fig. 70. — Diagrams of flower "sports" or mutations in the fireweed, 
Chamcenerium angustifolium. 

infrequence of mutants is taken into account in connection with 
these facts, it seems probable that origin by mutation plays but 
a minor part in evolution. 

210. Origin by hybridation. When two individuals more or less 
unlike are cross-pollinated, the result is a hybrid. When the parent 
individuals belong to the same form, crossing merely produces 
ordinary fertilization. If the plants are quite different, i.e., if 
they belong to distinct genera, the foreign pollen is as a rule unable 
to produce fertilization. Hybrids ordinarily arise between related 
species, between a species and its varieties, or rarely between 
varieties. In the first case crosses are said to be unisexual, i.e., 



198 PLANT PHYSIOLOGY AND ECOLOGY 

a certain character found in one parent does not occur in the 
other. In the second case the cross is termed bisexual, certain 
characters of the parents combining in pairs. In unisexual crosses 
the hybrid may show all the characteristic features of both parents, 
some individuals resembling one more than the other. The hybrid 
individuals may resemble one parent more than the other, even 
to the extent of being scarcely distinguishable from it. In bi- 
sexual crosses between a species with a certain character present 
or dominant and a variety of it, in which this character is latent 
or recessive, hybridation takes place in accordance with Mendel's 
law. All of the first generation of hybrid individuals show the 
dominant character of the species. If these plants are self -fertilized, 
approximately three fourths of the second generation show the 
dominant character, while one fourth exhibits the recessive charac- 
ter of the variety. If the flowers are again self-pollinated, the 
recessive individuals are found to come true to type. The domi- 
nant group splits, some of the plants remaining dominant while 
the others show the hybrid character of the preceding generation, 
i.e., their progeny will contain recessive, dominant, and hybrid 
forms. 

The production of new forms by hybridation occurs only when 
the resulting hybrids are in some degree a mixture of the charac- 
ters of the parents. It seems not to be a frequent source of evolu- 
tion in nature, though a few distinctive forms are known to have 
originated in this manner. 

Experiment 58. The occurrence of new forms in nature. Make a 
careful scrutiny of the species of the flora for the purpose of discovering 
ecads, variants, mutants, and hybrids. Note the differences between 
the parents and the new forms discovered. Estimate the chances the 
new forms have of surviving, using number, vigor, kind of modification, 
etc., as a basis for this. 

211. Natural selection. The success of some individuals and 
the handicapping or destruction of others in the process of natural 
selection is due either to the action of physical factors or to com- 
petition. Since the latter really operates through the reaction 
upon the habitat, natural selection rests finally upon the ability 
of a variation or change to cause a form to thrive in a particular 
habitat. In both mutation and hybridation, the new form appears 
suddenly, and selection can have nothing to do with its origin. 



THE ORIGIN OF NEW FORMS 199 

It merely determines whether the new form can persist in the 

place where it arises or to which it may be carried. In adaptation, 
natural selection and the process of adaptation go hand in hand 

and selection is obscured. Since an ecad is the direct product 
oi a habitat, it can not arise in a place unfavorable to it, while 
mutants and hybrids originate regardless of their fitness for the 
habitat. Natural selection consequently plays no part in pro- 
ducing new forms by either of these two methods of origin. On 
the other hand, it furnishes the only means of accumulating and 
preserving minute indefinite variations, and it plays a part in 
ad a in at ion. The question of its value in evolution must rest 
chiefly upon experimental evidence that new forms originate by 
variation. 

212. Isolation. As already indicated, the fate of any new form 
is determined not only by competition and the physical factors 
of the habitat ; it depends also upon the chances of crossing with 
the parent form or related forms. If forms do arise by variation 
and selection, the possibility of origin depends largely upon the 
absence of repeated inter-crossing. Forms which are prevented 
from crossing with each other are said to be isolated. Isolation 
may be due to physical or biological barriers, to distance, or in 
short to any condition which prevents the access of strange pollen. 

Isolation has often been thought a necessary condition for the 
origin of species. This can be true only of such forms as originate 
through variation. Minute differences between the various in- 
dividuals would be constantly leveled, and the cumulative action 
of selection would be effective only upon individuals cut off from 
the main group. In origin by adaptation and mutation, isolation 
naturally can take no part. Yet it does directly affect the per- 
sistence of either ecad or mutant by preventing crossing and 
consequent merging with the parent form. In so far as origin by 
hybridation is concerned, it is evident that isolation renders it 
impossible, owing to its complete dependence upon cross-pollination 
as a cause. 

213. Polygenesis. The origin of a form at two or more distinct 
places or times is known as polygenesis. Species were long sup- 
posed to have been created but once and in a single place. This 
idea of a single origin for all species was carried over into evolution, 
and Darwin maintained it vigorously against the doctrine of 
multiple origin. For many years after the general acceptance 



200 



PLANT PHYSIOLOGY AND ECOLOGY 



of evolution, the theory of single origin was retained. This was 
in spite of the fact that almost insuperable difficulties were often 
found in trying to explain through migration the presence of the 
same species in two or more remote and isolated regions. 

The view that the same species may arise at different places or 
times has recently been maintained by several ecologists. 1 It is 




Fig. 71. — The mountain fringed gentian, Gentiana amarella, illustrating 
polygenesis. The first dwarf grew in alpine gravel at 3700 m., the 
second in subalpine gravel at 2800 m. and only a few inches from the 
normal many-flowered form. 

at once evident that this may occur in the case of hybridation 
whenever the parents are spread over a wide area. The experi- 
ments of De Vries and his followers upon Oenothera have proved 
that the same mutant may arise at remote places as well as at 
1 Research Methods, 230. 



THE ORIGIN 01 m:\\ FORMS 201 

different times. The writer's own studies have demonstrated 
that this is equally true of ecads. In consequence, the conclusion 
is unavoidable thai it holds equally well for variants, since the 
conditions that bring about variation and selection would recur 
at various points in the area of a species widely distributed. 

A form that arises in two or more places is called jwlytopic, 
one originating at different times, polychrome. Contrasted with 
these are monotopic forms which originate but once. On account 
of migration, it is practically impossible to determine whether a 
species is polytopic or monotopic except by experiment. It is 
equally impossible to tell at present which method is the usual 
one, though it is probable that most forms are monotopic. 

214. Experimental evolution. Three fundamental methods 
form the basis of the experimental study of evolution, 1 which 
alone can yield trustworthy results. The first makes use of the 
actual experiments in adaptation, mutation, hybridation, and 
variation which are found in nature. The second produces similar 
experiments in nature, either by changing the habitat in which 
the form concerned is found, by transferring the plant to new 
and different habitats, or by actual crossing. The third method 
is a modification of the last, by which plants are brought into 
the greenhouse and subjected to known factors, which are kept 
under control. While these methods of experimental evolution 
seem simple, they are out of place in an elementary study of 
ecology. The origin of a new form is so complex and such thorough 
and painstaking study is required, that experiments of this sort 
must be left to the specialist. 

1 Research Methods, 149. 



CHAPTER X 
METHODS OF STUDYING VEGETATION 

215. The study of vegetation. It is desirable to study the 
effect of physical factors upon vegetation with the same care 
and thoroughness that are used in the case of the plant. As is 
shown more clearly later, vegetation responds to the habitat 
by means of changes and structures, which correspond in a general 
way to the functions and structures of the individual plant. It 
accordingly exhibits both adjustment and adaptation. In other 
words, it is possible to trace the development of vegetation and 
to study and record its structures. In order that these primary 
tasks may be carried out with accuracy and thoroughness, it has 
been necessary to invent methods which yield definite and detailed 
results. The latter are just as necessary for vegetation as for the 
plant. In fact the careful study of the habitat and plant loses 
much of its value if it is not also extended to the vegetation. 

Methods suited to the study of vegetation must make it possible 
to discover and follow the smallest changes and to recognize the 
innumerable details of structure. In addition they must be of 
such a nature that they furnish a complete and detailed record 
of all the changes and structures found. The quadrat method, 
with its modifications, meets all these requirements, and in con- 
nection with maps, photographs, and formation herbaria forms 
a complete system for the exact study of vegetation. 1 

216. The quadrat. As the name indicates, the quadrat is a 
square area of varying size marked off in the formation, i.e., one 
of the many units that make up vegetation. In its simplest form, 
it is used to count the individuals of each species, and to determine 
the relative abundance and importance of the species of a forma- 

1 Research Methods, pp. 160-198. 

202 



METHODS OF STUDYING VEGETATION 



203 



tion. In doing this, one discovers man}' points commonly over- 
looked, and gains a fair idea of the minute structure of a bit of 
vegetation. The quadrat is also used to follow the changing 
aspects of a formation during the growing period and to show the 
exact differences between diverse areas of it. In the chart quadrat 
the position of each plant is noted and recorded upon the chart, 
which thus becomes an indispensable record of structure, and a 
starting-point for making out future changes. Permanent and 




Fig. 72. — A quadrat with charting tape. 

denuded quadrats are modifications by which the exact study of 
an area is extended over a term of years. 

While a quadrat is but a small bit of a formation, it shows 
the exact structure of this bit. It is impossible to study the entire 
area with the same thoroughness, but a number of quadrats 
located with care in those places which appear different at a glance 
will reveal the entire range of structure. The quadrat, like 
any other method, must be used with discrimination, and not 
located at random. 

217. Kinds of quadrats. The unit size of the quadrat is a 
meter and this is the size of the quadrat commonly used. For 



204 PLANT PHYSIOLOGY AND ECOLOGY 

the sake of convenience, larger quadrats are square also. A 
major quadrat is a square of four units, and a per quadrat one of 
sixteen units, i.e., it is four meters square. Quadrats are also 
named with respect to the use made of them. A list quadrat is 
one in which the species are listed, and the number of individuals 
of each is counted. Chart quadrats are those in which the position 
of each plant is accurately indicated upon the chart of plotting 
paper. Permanent quadrats may be of either sort, though they 
are nearly always charted. They are distinguished by the fact 
that they are marked in a way to permit of study from year to 
year. The denuded quadrat is a permanent one, from which the 
plants have been removed in order that the manner in which they 
re-enter may be followed. 

218. Marking out quadrats. The tapes used in establishing 
quadrats are one or two meters long and a centimeter wide. 
They are divided into decimeter intervals by means of eyelets, 
and the intervals are numbered from left to right as conspicuously 
as possible. The tapes are held in position by means of wire stakes 
which hold the tape close to the ground, and have loops by which 
they are readily moved. 

In staking a quadrat the end tapes are always placed so that 
the numbers read from left to right, and the side tapes so that 
they read downward. In making a chart a fifth tape is stretched 
parallel to the top tape and a decimeter from it. When this 
strip is charted, the upper tape is moved to the next interval, and 
so on, thus permitting the rapid and accurate mapping of the 
whole quadrat. In all cases care must be taken to stake quadrats 
in such a way that they are square. 

219. The list quadrat. This is used when it is desired merely 
to obtain the number of individuals, i.e., abundance, usually in 
connection with the chart quadrat. The size of the list quadrat 
depends chiefly upon the nature of the vegetation. In herbaceous 
formations the usual size is the major quadrat which is two meters 
square, but when the plants are small and crowded the meter 
quadrat is used. 

In listing a quadrat, i.e., counting the number of individuals 
of each species, the smaller, less conspicuous plants are listed first, 
since these are apt to be tramped down. When the outside 
tapes and the taller species afford sufficient landmarks, a single 
species is counted at a time. Otherwise a fifth tape is used to 



METHODS OF STUDYING VEGETATION 205 

mark out each decimeter strip, and the plants are checked off 

as they are found. Except in cases of unusual difficulty, plants 
should never be broken or pulled as they are counted. Clusters 
and hunches of stems from the same root are counted as single 
plants, and the number of stems indicated by an exponent. In 
the case of bunch-brasses, each bunch is counted as one plant. 

220. Abundance. Species are arranged in the quadrat list 
upon the basis of their abundance, i.e., number of individuals. 
They are also divided into groups which correspond to the various 
degrees of abundance. Two types of abundance are recognized, 
owing to the fact that the individuals of some species occur in 
groups, while those of others are more or less uniformly arranged. 
The former are said to be gregarious, the latter copious. The 
species counted are classified with respect to the following table 
of abundance: 

Social exclusive, no other species of vascular plants present. 

Social inclusive, more than 100 in the quadrat. 

Gregarious 1 gr 1 100-50 Copious 1 cop 1 

Gregarious 2 gr 2 50-25 Copious 2 cop 2 

Gregarious 3 gr 3 25-10 Copious 3 cop 3 

Subgregarious sg 10-5 Subcopious sc 

Vixgregarious vx 5-1 Sparse sp 

While the number of plants per quadrat gives a much clearer 
idea of the relative importance of the species than the usual terms 
abundant, common, rare, etc., height and w r idth have much to do 
with the question of importance. The part which a species plays 
in giving character to vegetation and its relation to the habitat 
can be determined only by taking into account the space it 
occupies, as well as its abundance. This may be done with sufficient 
accuracy, after finding the average height and width of the plant 
body of any species, by means of the formula, height (xR 2 ) X abun- 
dance. 

221. The chart quadrat. Whenever it is desired to obtain an 
exact record of changes of structure, the chart quadrat is used. 
The meter quadrat is preferable on account of the labor involved 
in charting. The location must be decided by the area to be 
studied and by the facts to be brought out. Chart quadrats are 
used chiefly for comparing representative areas of different forma- 
tions or diverse areas of the same one, as well as the spring, 
summer, and autumn aspects of the latter. 



206 



PLANT PHYSIOLOGY AND ECOLOGY 



222. Making quadrat charts. The quadrat is staked out in the 
manner already described. The chart is made to the scale of 
10:1. A square decimeter is outlined on centimeter plotting 
paper, and the centimeter squares are numbered at the edges to 
correspond to the intervals of the quadrat. The upper and lower 
lines are numbered from left to right, and the side lines from top 
to bottom. Mapping is always begun at the upper left-hand 
corner of the chart. The position of the plants in the first deci- 
meter of the quadrat is indicated in the first centimeter of the 




Fig. 73. — -A quadrat in the foothill thicket formation near Manitou. The 
principal species is the painted-cup, Castilleia Integra. 

chart, the small squares aiding in determining the exact location. 
As soon as the first decimeter strip is plotted, the upper tape is 
moved to outline a new strip, and this is repeated until the quadrat 
is finished. 

Each plant is put down whenever possible, but mats, turfs, 
and mosses are merely outlined in mass as a rule. This is usually 
done with large rosettes and mats also, even when they are single 
plants. Each plant is represented by the initial letters of the 
name. The first letter of the generic name is used, if no other 
genus found in the quadrat begins with the same letter. If two 



METHODS OF STUDYING VEGETATION 



207 



or more genera begin with the same lot tor, e.g., Agropyrum, Allium, 
one, the one most abundanl is indicated by a, and the others 
by the first two letters, as al, an. In case two species of the same 
genus are present, the species initial is combined with the generic 
one, e.g., ac and ar for Agropyrum caninum and A. richardsonii 
respectively. When a similarity in names would require three or 









l 










•^ 










3 










1 










■ r ) 










G 










7 










s 










B 










1U 
















































































































































a 
























I 






































1 
















































































































































































































































































































































































































































































' 








































1 


















. 














































































a 






















































>l 
































































































































as 




































































































































































































































3 












































































































. 




































































































a 


1 




























































a 








































































































































■a 








































c' 


























































4 


















































































































































































































a 




























































































































































































































. 


















<' 


























































B 














































































































































































































a 


/' 












































































<r 






























































































































































































































€ 


























I 






































































































































































































































1 


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if 






















ar 


































































































































































































7 
































































































c 4 




























































































































































































































































































































































































































8 






















































































































































































































































































































i 








































































































































































































1 








































































































































































































































































































































































































































































n 


n 




















































o 






































































































































„ 







































































































































































10 



Fig. 74. — Chart of the quadrat shown in Fig. 73. The principal species 
are Castillcia Integra (c), Artemisia frigida (a), and Aragallus lamberti 
(ar). 

more letters, e.g., Androsace, Anemone, Antennaria, this is avoided 
by fixing an arbitrary sign for one, viz., at. The number of stems 
from one root is indicated by an exponent with the proper initial, 
viz., a 3 . Seedlings are represented by a line drawn horizontally 
through the letter. Plants in flower or fruit are distinguished' 
by a line drawn vertically through them. In charting the seasonal 



208 



PLANT PHYSIOLOGY AND ECOLOGY 



aspects, however, the rule is to indicate only the characteristic 
species, i.e., those that flower at the time concerned. The legend 
giving the list of abbreviations and species is placed directly 
below the chart. Each chart is numbered, and the formation, 
place, and date indicated. When physical factors are deter- 
mined for the quadrat, these are recorded upon the chart in so 
far as possible. 

223. The permanent quadrat. Chart quadrats which are 
marked so that they can be visited and studied from year to year 
are permanent quadrats. The latter are indispensable for follow- 




Fig. 75. — A permanent and a denuded quadrat in the gravel slide forma- 
tion. Both quadrats were charted and one then denuded. 



ing the changes of aspects with the season and the slow yearly 
changes which formations undergo in their development. Since 
they record the existing structure as well as its subsequent changes, 
permanent quadrats are used almost exclusively in preference to 
list and chart quadrats. Practically all formations are constantly 
undergoing more or less change, much of which is so slow or obscure 
that it can be discovered only by the permanent quadrat. New 
plants are entering through invasion, others are disappearing in 
consequence of it. The extent and rapidity of such changes can 
be ascertained only by the minute and repeated study of a definite 
area. This is especially true where one formation is being replaced 



METHODS OF STUDYING VEGETATION 209 

by another, Le., in the process called succession, in which the use 
of the permanent quadrat is imperative. 

The permanent quadrat is a meter square. It should be located 
in a station where readings of physical factors arc taken. If such 
quadrats arc established elsewhere, readings should be made in 
them in so far as possible. Permanent quadrats are staked and 
mapped in exactly the same way as chart quadrats. The quadrat 
is fixed by driving a labeled stake at the upper left-hand corner, 
BO that its edge indicates the exact position of the quadrat stake. 
A -mailer one is placed at the opposite corner to facilitate the 
task of setting the tapes accurately in later readings. The label 
-take bears merely the number of the quadrat and the date when 
it was first established. It is driven in firmly and is allowed to 
project just enough to enable it to be re-located with readiness. 
The use of natural or artificial landmarks is necessary in order 
that the stake may be found easily upon successive visits. At 
each subsequent visit the tapes are placed with reference to the 
stakes, and a chart is mapped in the usual manner. These are 
labeled and dated like the original ones, but they are numbered 
to indicate both the quadrat and the visit, e.g., 15 2 is the second 
chart made of quadrat 15. 

224. The denuded quadrat. This is ordinarily a permanent 
quadrat, from which the plant covering has been removed after 
a chart and photograph have been made. Practically the same 
thing is obtained by staking a permanent quadrat in a new soil 
or in one recently laid bare. The denuded quadrat is of the usual 
size, 1 meter. It is especially adapted to the study of invasion 
and the resulting competition, and throws a flood of light upon 
the development of formations in the course of succession. 

Permanent quadrats may be denuded at any time that seems 
desirable. The best practice is to establish two side by side, 
and then denude one of them, the other serving as a control. A 
quadrat which is to be denuded is first mapped, photographed, 
and labeled exactly like a permanent one. The vegetation is then 
destroyed, usually by removing it with a spade. Ordinarily the 
aerial parts alone are removed by paring the surface of the ground. 
When it is washed to trace the consequences of a greater disturbance,, 
the upper seed-bearing layer of soil is removed and the underground 
parts dug up. Quadrats are usually denuded in the fall, at or 
near the close of the growing period, though it may also be done 



210 PLANT PHYSIOLOGY AND ECOLOGY 

in the spring. In this case invasion is ordinarily delayed, owing 
to the removal of accumulated seeds and propagules. The treat- 
ment of denuded quadrats upon succeeding visits is the same as 
for permanent ones. Since denuding practically makes a new 
habitat, the factors which control invasion can be found only 
by taking readings within the denuded area. Such readings are 
of the greatest value when they can be compared with those of 
an adjoining permanent quadrat. 

225. Transects. The transect is a cross-section of vegetation. 
It is practically an elongated quadrat, extending through a station, 
a formation, or a series of formations. Unlike the quadrat, it is 
designed to show in a graphic manner the differences in structure 
between two or more contiguous areas. While the quadrat is 
always located in a homogeneous area, the transect traverses areas 
more or less unlike, and is plotted with especial reference to topog- 
raphy. The transect is principally used to bring out the differences 
between zones, and between those groups of individuals called 
societies, communities, and families. With respect to dimension, 
transects are distinguished as line, belt, or layer transects. Belt 
transects are permanent and also denuded. The last belongs 
properly to investigation and hence is not discussed here. 

226. The line transect. This is the form employed when only 
the more striking differences in structure are sought. It is espe- 
cially adapted to elementary study, on account of the relative 
ease with which it may be run. It is ordinarily used for an entire 
formation or for a series of them. A simple line transect is made 
by establishing the desired points and then recording the plants 
pace by pace along the line between them. A more accurate 
method is commonly used to give detailed results. In this tapes 
exactly like quadrat tapes but 10, 50, or 100 meters long are used. 
The transect is located in the area to be studied by running the 
tape from one landmark to another, and fastening it here and 
there by quadrat stakes. When the topography is not level, it 
is necessary to obtain the length and angle of the slopes in order 
that an exact outline map may be constructed. In noting 
the plants that occur along the tape, every second vertical line 
on the centimeter plotting paper is taken to correspond with the 
tape. The individual that touches the latter is recorded to the 
right or left respectively, and within the centimeter square that 
corresponds to the particular decimeter interval of the tape. When 



METHODS OF STUDYING VISITATION 



211 



it is desirable to save the time, 
the plants are noted on either 

side alone. The species are 
indicated by initials as in 
quadra! mapping. In plotting, 
the topography is carefully 

drawn to scale, and the rows 
of initials transferred from the 
field record to the outline, 
centimeter by centimeter. A 
10-meter transect can thus be 
recorded on a meter sheet upon 
the scale of 10:1. Transects 
longer than this are drawn to 
a scale of 100:1 or 1000 :1, and 
the details must be correspond- 
ingly reduced. When both 
detail and length are desired, 
they may be secured by divid- 
ing the line into 10-meter 
parts and assigning one part 
to each student. 

227. The belt transect. 
This is a belt instead of a line, 
and its width consequently 
permits a more detailed and 
accurate record of the arrange- 
ment of the plants. The 
width of a belt transect is 
determined by its length and 
by the character of the vege- 
tation. The usual width is one 
decimeter in herbaceous for- 
mations and one meter in 
woodland formations, when 
only the trees and shrubs are 
taken into account. 

In staking a belt transect 
two tapes are employed to 
mark out a strip just one 







Fig. 76. — A line transect through a bog 
formation which has invaded a forest 
along a brook. The ecotones between 
the two formations are shown at e. 



212 PLANT PHYSIOLOGY AND ECOLOGY 

decimeter wide. The distance between them is checked here and 
there by a decimeter rule, and they are fixed firmly in place by 
quadrat stakes. The plants are recorded as for the line transect, 
except that the record is for a decimeter strip, and occupies the 
width of a centimeter on the plotting paper. An interval of a 
centimeter is left between the successive portions of the strip, in 
order that they may be copied readily upon the topographic 
outline. The latter is traced on the plotting paper as indicated 
for the transect, except that it consists of two lines a centimeter 
apart. The outline and the field record of the plants of the tran- 
sect are combined upon a common scale, as in the line transect. 
Because of their value and the labor involved in making them, belt 



■ -.-"■"■"■■; - 


" 


-V - I 


' ^v-l^^'v 7 






- 'UL J. J i -*K~^ 


• ■■ »$m.im2 i '<■ 


^■■■ippii^^.^ 


' 




' 5 - ; :;*5 








--"W* 


'■ -■?.*' " ■-■ K ? ' V x .« 


■ 







Fig. 77. — A belt transect through the Fragaria society of a spruce forest. 

transects are regularly made permanent by placing a labeled stake 
at each end. 

The limits of zones are shown on charts of belt transects by 
single cross-lines, and those of alternating areas by parallel cross- 
lines. Whenever possible, the physical factors should be deter- 
mined for the various areas through which the transect runs. 

228. The migration circle. This is the method employed to 
show the movement or migration of plants outward from a 
parent individual or group. Such a movement usually takes 
place in all directions, and a circle is hence better to record it than 
a quadrat. Migration circles are regularly permanent in order 
that the yearly spread of the plants may be accurately followed. 
Circles of this sort are of great value in the study of competition 
and invasion. The actual movement of the migrants is often 
seen to the best advantage in denuded circles, but these are scarcely 
feasible except for protracted investigation. 



METHODS OF STUD1 [NG \ EGETATION 



213 



A simple migration circle is located so thai a plant or group 
of plants oi the species to be studied forms the center. The Bize 
of the circle is largely determined by the nature of the vegetation, 
and especially by the height of the species, which lias much to 

do with the distance to which seed or fruit is carried. A circle 
of 1 -meter radius is best for ordinary purposes, though in open 
vegetation one of 5-meter radius is desirable. The usual quadrat 
tape is used as a radius. This is fixed in the center and the position 
of each plant of the species concerned is noted as the tape is carried 




Fig. 78. — A migration circle in a grass formation, used to determine the 
direction and amount of movement of the achenes of Kuhnia glutinosa. 

around the circle. The exact positions are indicated upon a chart 
circle whose radius is a decimeter. The chart is ruled in such a 
way that each quarter is divided by five radii, in order to aid in 
recording the individuals accurately and quickly. After the 
mapping is completed, a labeled stake is fixed in the center, thus 
making the circle permanent. It is often desirable to use a group 
of individuals of different species as a center, and to record the 
movements of all upon the same chart. 

229. Formation maps. In studying the structure of a forma- 
tion, it is important to make a graphic record of its principal 



214 PLANT PHYSIOLOGY AND ECOLOGY 

features. This is most readily done by means of an outline map 
in which the various zones, consocies, communities, etc., are 
shown. Sufficiently reliable data for such maps can be obtained 
by pacing. In securing measurements for the map of a formation 
or some part of it, it is first necessary to select a base. This may 
be a road, ravine, ditch, pool, lake, or stream, or the peak or crest 
of a hill, ridge, or mountain. The width and length of the base 
are determined, as well as its general direction. The width and 
length of the various areas, zones, consocies, etc., are then measured, 
together with the distance and direction of each from the base. 
These are reduced to the scale desired, viz., 100:1 for small areas, 
and 1000:1 for larger ones. The base is first outlined upon a 
meter sheet, usually of plotting paper, and the various areas are 
then drawn in the proper size and position. Each area is labeled 
to denote its character, or it may be colored, thus causing the 
various parts to stand out more distinctly. 



CHAPTER XI 

THE PLANT FORMATION 

230. The nature of formations. A formation is an area of 
vegetation, such as a meadow, a forest, a prairie, a bog, a cliff 




Fig. 79. — A lichen formation on the face of a cliff (Lecanora and Umbilicaria). 

covered with lichens, or a pond of water-lilies. Its limits may 
be sharply defined, as is true of the forest, cliff, and pond, or one 
formation may pass almost insensibly into another. For example 

215 



216 PLANT PHYSIOLOGY AND ECOLOGY 

a meadow may pass so gradually on the one side into a swamp 
and on the other into a prairie that it is impossible to say exactly 
where the one stops and the other begins. Nevertheless, meadow, 
prairie, and bog are three different formations, as is readily seen 
when areas typical of them are compared. In consequence, the 
real test of a formation is its character or composition rather than 
the sharpness of its limits. Adjacent formations of the same 
general nature usually shade gradually into each other, e.g., 
meadow and prairie, forest and thicket, etc. Those that are very 
different in character, e.g., meadow, pond, and forest, stop abruptly 
at the line of contact. 

231. Recognition of formations. The formation is the unit 
of vegetation. The plant covering of the earth is a vast complex, 
largely made up of different formations. In recognizing these 
units or formations, one or two precautions are necessary. The 
unit itself shows parts which may be mistaken for formations. 
This danger is considerable when a formation has been so broken 
up by natural or artificial forces that one or more of its parts have 
been separated from the original. The chance of confusion is 
greater when the original formation has disappeared from a region, 
leaving only one or two incomplete parts to represent it. In 
such cases the most careful study is necessary to determine the 
proper standing of these isolated areas. The need of caution is 
well illustrated in forested countries that have been largely cleared, 
and in grassland regions which have been almost wholly plowed up. 
Fragments of the original forest or prairie are left here and there, 
all more or less widely separated. Some of these are so different in 
composition that they appear by comparison with each other to be 
distinct formations. When these are all brought together, and 
especially when they are compared with larger areas in other 
places, they are found to be merely the more or less different 
portions of an original formation. In studying small areas of a 
few square miles or less, it must constantly be kept in mind that 
these are probably not different formations, but merely parts of 
an originally extensive formation which have now become 
separated. This is especially true of rugged regions, such as 
mountains, in which the pieces of one formation are very small 
and widely scattered. 

Similar care is necessary in regard to areas which show different 
stages of development. One stage or formation changes slowly 



THE PLANT FORMATION 



217 



into the succeeding one, and during the process the vegetation 
is really a mixture of the two. The same formation often appears 
on several new or denuded soil areas. In some of these it is com- 
posed of entirely different species, apparently indicating a number 
of distinct formations. If a critical comparison is made of all 
such areas, or their further development traced, it can be determined 
whether they are parts of one formation. 

One part of a formation may lag behind the others in develop- 
ment, or a physical change or difference covering a small area 




Fig. 80. — A cliff formation of saxifrage herbs and 
bushes (Heuchera and Edwinia). 

produces an alien group in a formation otherwise uniform. The 
lichen and moss groups that are found on rocks in forest and grass- 
land furnish a good illustration of this. The lichens which grow 
on the forest floor and those which are found on the trees clearly 
belong to the forest formation. They are present because of the 
shade and moisture furnished by the trees. The same is true of 
those found among the grasses and sedges of meadows. The 
physical conditions are those of the formation, and the lichens 
are as much at home as the other plants. In the case of rocks 



218 PLANT PHYSIOLOGY AND ECOLOGY 

scattered through forest or meadow, the physical factors are 
changed, and the species growing on rocks are found to be typical 
of rock formations elsewhere. Each rock group is a fragment 
of a formation which is characteristic of a different habitat. The 
rock may have found its way by accident into the forest or meadow; 
it may have been uncovered and thus come to serve as a stratum 
for the lichens, or it maybe a relict of a rock formation that has 
been displaced by other vegetation. In either event the lichen 
group is foreign to the forest or meadow. 

From the above it is clear that the first task in field work is 
to distinguish as many different areas as possible in vegetation. 
The next and more important task is to compare the composition 
and development of these so carefully that the actual formations 
may be recognized, and the various pieces referred to the proper 
one. In any region the number of formations is very small in 
comparison with the number of parts, fragments, developmental 
stages, etc. It is a simple matter to recognize a forest, meadow, 
bog, or pond as formations in a locality, and very often the forests, 
meadows, etc., of the neighboring localities are merely different 
examples of the same formation. 

232. Relation between habitat and formation. Since forma- 
tions are groups of individual plants and these are dependent upon 
the habitat, it is evident that the habitat must have the same con- 
trol over the formation. Strictly speaking, a formation is the mass 
of plants which cover a habitat. The limits of the two are neces- 
sarily the same. A habitat with sharply defined limits, e.g., a 
pond, a rock, a shaded area, is occupied by a formation whose bound- 
aries are equally definite. One that shades imperceptibly into 
another shows a formation that grades into the adjoining one. 
The real characteristic of a habitat is a striking difference in one 
or more of the direct factors, water content, humidity, or light. 
A series of habitats thus set off from each other bear characteris- 
tically distinct formations, as, for example, xerophytic vegetation 
on a gravel slide, shade plants in a forest, and bog plants in a 
wet meadow. 

The formation is the product of its habitat. It not only shows 
a general and typical response to the latter, but it also conforms 
to the minor differences which occur in it. The conditions of 
water content and light that produce a spruce forest cause it to 
be sharply set off from a gravel slide formation produced by very 



THE PLANT FORMATION 219 

different amounts of water and light. Within each habitat, 
however, these factors vary more or less, and their variation is 
reflected in the format ion. In consequence, the formation shows 
corresponding differences in composition, and may be analyzed 
into a number of different parts. The latter are often charac- 
terized by different species, which give them a more or less dis- 
tinct stamp. Close analysis shows, however, that these are all 
of the same general nature, and that the arrangement is a response 
to the minor variations of physical factors or to competition. 

233. The historical factor. In many instances the variations in 
the arrangement of species and individuals are due to historical 
reasons instead of physical ones. An invading species may enter 
the formation at one point and spread slowly from this. It rarely 
or never happens that it spreads quickly and uniformly in all 
directions. In the development of vegetation, one or more species 
or individuals may persist as relicts for a long time after their 
fellows have disappeared. This may be due to the accidents of 
migration and competition, or to the fact that the plant itself 
has a certain ancestral or historical quality that enables it to 
persist. This historical element also explains why distant portions 
of a large formation may show differences that are independent 
of habitat. It also throws much light upon the puzzling changes of 
various isolated areas of it, since these are especially exposed to 
invasion. In regions remote from each other, similar habitats 
are occupied by similar formations, but the species concerned are 
largely or entirely different, owing to historical facts of migration 
or development. 

234. Development and structure. Formations show certain 
changes and certain differences in composition. These may 
properly be called activities, or functions, and structures. The 
formation itself may be regarded as a complex organism which 
shows both development and structure. Practically all forma- 
tions are undergoing constant change. These changes are rapid 
in the period of development, but are slow and almost imper- 
ceptible after the formation becomes stable. They are charac- 
teristic of young formations, but occur in small degree in old 
ones. 

The changes which make up development are brought about by 
the movement and establishment of plants, by their reaction upon 
the habitat, and by the competition between them. Accordingly, 



220 PLANT PHYSIOLOGY AND ECOLOGY 

the activities of groups of plants are aggregation, migration, estab- 
lishment, reaction, and competition. Migration and ecesis con- 
stitute the basis of invasion, while reaction and competition are 
typical of the complete invasion which causes one formation to 
replace another, i.e., succession. 

In conjunction with the habitat, these functions produce and 
modify the grouping of the individuals and species of the forma- 




Fig. 81. — A willow thicket formation, consisting of several species of Salix, 
just above timber line on Pike's Peak. 

tion. As has already been seen in the case of the plant, the activi- 
ties of the formation furnish the clue to its structures. The latter 
are more or less definite areas produced by variations in the habitat, 
by the activities of the formation, or, in the majority of cases, by 
both. In addition formations are themselves grouped together 
in such a way that vegetation itself shows a more or less definite 
structure. In both cases, two distinct types of arrangement or 
structure may be distinguished. In the one the parts of forma- 
tions or the formations themselves are arranged in zones, and the 
type of structure is called zonation. In the other the areas show 



THE PLANT FORMATION 221 

do regular order, but seem to fit together in a haphazard fashion, 
which is termed alternation. 

To avoid repetition, the development of the formation is treated 
together with that of vegetation in the three chapters that follow. 
Since the formation is the working unit in the study of vegetation, 
its structure and classification are taken up in detail here, while 
the general discussion of zonation and alternation is reserved for 
the final chapter. 

235. Structure of the formation. No formation is uniform 
throughout its entire extent. Practically all show more or less 
striking differences at every step, so that minute and universal 
variation may be regarded as a law of formational structure. 
This characteristic variation is the result of the action of physical 
factors and of formational activities, i.e., migration, competition, 
etc., upon the number and grouping of individuals and species. 
It finds expression in three ways. Species come to be distinguished 
from each other with respect to number and to their importance 
in the formation. They adjust themselves to seasonal changes 
in such manner that they appear only at a certain time, or are 
more characteristic at one season than at another. Finally the 
variations within the habitat, together with aggregation, migration, 
and competition, arrange individuals and species in more or less 
typical groups or areas. 

236. Facies. The importance of the part which each species 
plays in the formation is largely determined by the number of its 
individuals. Other factors also have considerable influence in 
determining this matter. It has already been pointed out that 
the size, i.e., height and width, of the individuals is an important 
factor. In addition other qualities of the plant, especially its 
duration, are of much importance. In determining the control 
which a species exerts in the formation, all of these points are 
taken into account, though in many cases number alone affords 
a satisfactory basis. 

The dominant or controlling species of a formation are termed 
facies. These are, as a rule, the most abundant, or they make up 
in size or duration what they lack in number. They are the most 
widespread species of the formation, though they are not neces- 
sarily found everywhere in it. In forests, the facies are the domi- 
nant species of trees; in thickets, of shrubs; and in grassland, 
they are the controlling grasses or sedges. When grasses are of 



222 



PLANT PHYSIOLOGY AND ECOLOGY 



little importance or absent, as in deserts, wastes, denuded areas, 
bogs, ponds, etc., the facies consist of other herbaceous plants. 
In meadows, prairies, and plains, however, the rule is almost in- 
variable that the dominant grasses alone are the facies. The 
latter are not necessarily the most conspicuous species of the 
formation. This is true in the case of forests and thickets, but in 
meadows and prairies other herbs often overtop and conceal the 
grasses. This is usually true for only a part of the growing period, 





. :. . 




Fig. 82. — The spruce forest formation (Picea and Pseudotsuga) at 
Minnehaha, near Pike's Peak. 

and at all other times such species are subordinate to the grasses, 
which always form the groundwork of the formation. 

237. Principal and secondary species. The rank of the remain- 
ing species is determined largely by their abundance, with some 
reference to their size. The most abundant and characteristic, 
the facies excepted, are termed principal species, while those of 
less importance are secondary species. The line between the two 
groups must be fixed more or less arbitrarily. Abundant con- 
spicuous species can be readily referred to the first group, just as 
sparse and inconspicuous ones belong to the latter. Between 



THE PLANT FORMATION 



223 



these lie the species of more or less importance. The order of 
these is determined by their abundance, and the place at which 
the line is drawn between principal and secondary species must 
be decided with reference to the species concerned. 

238. Aspects. The general appearance of a formation changes 
more or less with the season. It usually has a different stamp 
during each season, and is said to show seasonal aspects. These 
are determined by the principal species which bloom at the season 
concerned, and give a particular impress to it. The facies take 





,£y- r.rt ' ' '<•' •■ 








fe;. '-■■ 



Fig. 83. — Early aspect of the alpine meadow formation, characterized by 
Rydbergia grandiflora. 



practically no part in this in the case of woody formations, where 
the seasonal change affects only the undergrowth. This is true 
to some extent in grasslands in which the facies are more or less 
obscured by taller herbs, though the flowers of the grasses play 
an important part in one of the later aspects. 

The growing period is usually divided into four aspects, cor- 
responding to the periods of flowering. These are the early spring, 
the spring, summer, and autumn aspects, which are also called 
prevernal, vernal, estival, and autumnal. In high mountain 



224 



PLANT PHYSIOLOGY AND ECOLOGY 



regions the period of growth is so short that only two aspects 
appear, the early and the late. Since the period of flowering 
fluctuates from year to year and since some species bloom for a 
long time and others for a short one, it is impossible to set accurate 
limits to each aspect. Each one passes more or less gradually 
into the next, the first summer flowers appearing before the last 
spring blossoms have gone. Nevertheless, a careful study of a 
formation throughout the year will make it clear that the plants 



!Mi^!M^iftTiJtl^^WMIiy JULiUflUiLMm HBtSJSQK^H 1 NB^f? j| mph^bbi 


*-/ '4-M.i 




■-•tV' ; •'■ - .-• ." : - 


i'-*^ • *• ■■>■■ : -' *: ' : X "' - 



Fig. 84.— Late aspect of the alpine meadow formation, characterized by 
Campanula petiolata. Rydbergia is seen in fruit. 

that flower in the spring are nearly all different from those of the 
summer, while the summer aspect contains few flowers that 
appear also in the next one. As a rule, the prevernal aspect 
includes the few early flowers that appear about the first of April. 
The spring aspect comprises April and May; the summer, June, 
July, and the early part of August; and the autumn aspect the 
remainder of the year. 

Experiment 59. Study of abundance and of aspects. Select two 
representative areas in a prairie or meadow formation. Locate a 
quadrat in each and list the various species, indicating the abundance 



THE PLANT FORMATION 



225 



of each. Arrange them in the order of abundance as indicated in sec- 
tion 219, and decide which arc fades and which principal or secondary 
species. 

If the quadrat is listed in the early spring, then marked and listed 
late in May or early in June, the prevernal and vernal aspects may be 
compared. When it is possible, it should be listed again at the end 
of July, and finally in September for the summer and autumn aspects. 

239. The parts of a formation : the consocies. The hetero- 
geneity of a formation is due to parts of varying rank which are 

















































































'jtMAmrm 


mm 








"J\HEj 



Fig. 85. 



-An area controlled by the rye grass, i.e., an Elymus consocies, 
in the Elymus-Muhlenbergia formation. 



called forth by historical as well as by physical factors. These 
parts or divisions, in the order of rank, are the consocies, the society, 
the community, and the family. In formations controlled by two 
or more fades, the latter are very rarely if ever uniformly dis- 
tributed. One will be more dominant here, another there, and 
a third elsewhere, or certain ones may be grouped together in 
one place, and others in another. The various areas that are 
controlled in this way by the facies are termed consocies. In 
the grass formation of the prairies there are several facies, viz., 



226 



PLANT PHYSIOLOGY AND ECOLOGY 



Bouteloua, Andropogon, Koelera, Stipa, etc. In certain places 
two or more of these may meet on nearly equal terms to form a 
consocies ; e.g., a Bouteloua-Kcelera-consocies. More frequently 
a ridge or slope is controlled by one facies, resulting in an Andro- 
pogon-consocies, a Stipa-consocies, etc. In some formations 
consocies are definite, in others they overlap or are indistinct, 
but in all that possess two or more facies they can usually be 
recognized. In grassland, however, they are often obscured by 
the more conspicuous groups of principal species. 

240. The society. An area characterized by a principal species 
is a society. Unlike the consocies, societies usually do not cover 



fcjfttefc 



TMpsW 4? ~ r , ' 









Fig. 86. — An Iris society, characteristic of the spring aspect of the aspen 
formation in the Rocky Mountains. 



the entire formation, but are separated from each other, the gaps 
being filled by secondary species and by the scattering individuals 
of principal ones. Societies moreover change with each aspect, 
as the principal species of one season replace those of the preceding 
one. In the prairie formation, for example, three characteristic 
societies of the spring aspect are the Astragalus, the Comandra, 
and the Lomatium societies. In the summer aspect these have 



THE PLANT FORMATION 



227 



disappeared from view, and Amorpha, Erxgeron i Kuhnistera, and 
PsorcUea societies have taken their place. The latter in turn 
become inconspicuous as they stop flowering, and the character 
of the autumn aspect is given by societies of Aster, Ladniaria, 
HdiarUhus, Solidago, etc. A society, moreover, is often char- 
acterized by two or more principal species. Societies have no 
essential connection with consocies. In any aspect, a consocies 
may include several or many societies, or it may not show a single 
one. Finally, a society may lie in two consocies, or it may occur 
in any of them. 

241. The community. An inspection of plant societies shows 
that they are far from uniform. This is of course true of any 




Fig. 87. — An Antennaria community of the aspen formation. 



intervals that occur between them. In both places either principal 
or secondary species may form minor groups called communities. 
In many cases these are fairly definite and may be readily recognized. 
In some instances, however, the number of species is so large and 
the arrangement of the individuals so varying that distinct groups 
are lacking. Communities are easily recognized in the case of 
species where individuals grow T in groups, and especially where 



228 



PLANT PHYSIOLOGY AND ECOLOGY 



families have been invaded by plants of another species. A large 
number of prairie and meadow species occur in more or less definite 
communities, e.g., Anemone, Draba, Lithospermum, Rosa, Laciniaria, 
etc. Communities appear in nearly all formations, but they are 
most abundant in the open ones which develop in new or denuded 
habitats. 

242. The family is a group made up entirely of individuals 
belonging to a single species. In many instances it springs from 
a single parent plant, but this is not necessarily the case. In size 
a family may consist of a few individuals or it may cover a large 
area. It passes into a community as soon as one or more individ- 




Fig. 88. — A Senecio family in the rock clefts on Pike's Peak. 



uals of another species enter it. In consequence, families are 
usually small, since they are readily invaded when large. Species 
whose seeds are numerous and little or not at all modified for 
dissemination often form families. For this reason annuals 
are found to form families much more frequently than perennials 
do. Families are more or less characteristic of new or denuded 
soils, in which each pioneer usually serves as a center for its off- 
spring. As the individuals become more and more numerous, 
they invade the neighboring families formed by other species 
and change them into communities. In the ordinary vegetation 
of forest and grassland, families are relatively rare, since they are 
readily turned into communities by the movement of their crowded 



THE PLANT FORMATION 



229 



neighbors. Occasionally a vigorous weed invades and by strong 
competition conquers a small area. This frequently happens 
when the original plants are destroyed over a small space, thus 
permitting some annual to enter and increase rapidly. I/ordewn 
and Lepidium families are especially apt to appear in this fashion 
in grassland. 

Experiment 60. The structure of a formation. Examine a prairie, 
meadow, or forest for families and communities. Make a chart of 
each and note the differences between them. 

Run a line transect through the formation from east to west, and 
north to south by pacing. Note the families, communities, societies, 
and consocies encountered. 

243. Layers. Forests and thickets show a more or less definite 
arrangement of the plants below the facies into layers. There is 




Fig. 89. — The Erythronium layer in the early spring aspect of the oak- 
hickory forest at Lincoln. 

also a suggestion of the latter in many grassland formations, while 
some thicket-like formations of tall herbs show much the same 
conditions as ordinary thickets. Layers are the result of the 
habit of growth of the various species, and this is largely a response 



230 PLANT PHYSIOLOGY AND ECOLOGY 

to the reduced light of the habitat. Tall-growing plants which 
require the most light form the first layer below the f acies, while 
the low forms which will grow in very weak light make up the 
lowermost layer. The number of layers in a forest depends upon 
the compactness of the primary layer of facies. When the light 
beneath the latter is reduced to .002 or .001, layers are impossible, 
since practically no flowering plants can grow under these con- 
ditions. Layers are developed to the highest degree when the 
primary layer permits more or less sunlight to shine through it, 
i.e., in a light intensity varying from .1 to .01. There is often a 
more or less incomplete secondary layer of shrubs and small trees. 
Below this usually occur two or three herbaceous layers. The 
upper one consists of tall plants about 2 meters high. It is followed 
by a middle layer about 1 meter high, in which the bushes are 
usually found, and the latter by a lower layer of small herbs 
2-5 decimeters high. Beneath these is found a ground layer of 
mosses, lichens, cup-fungi, toadstools, etc., which is the only one 
that remains in the densest forests. The herbaceous layers are 
always more or less interrupted, owing to their close dependence 
upon light. Either one of them may be absent, or finally all of them 
may disappear, as has been indicated. 

The explanation of layers, as well as that of consocies and 
societies, depends upon a knowledge of zonation and alternation 
and is to be found in the chapter dealing with these principles. 

Experiment 6i. Layered formations. Make a careful examination of 
a forest or thicket, noting the number and extent of the layers present. 
Ascertain the characteristic species of each layer and note the various 
groups which they form in it. 

244. Classification. Formations are classified with respect to 
habitat, development, position, dominant species, or their general 
character. Classification upon the basis of habitat places together 
formations which are similar in their response to physical factors 
and in general structure. Developmental classification is based 
upon the fact that the formations which follow each other in the 
development of vegetation upon a new or denuded area have a 
certain organic relation to each other. In many cases it also 
brings out certain important relations to physiography. Grouping 
with respect to position is based solely upon geographical factors. 
The formations brought together in this way have little relation- 



THE PLANT FORMATION 



231 



ship to each other beyond the superficial one of location. Forma- 
tions, especially contiguous ones, sometimes show more or less 
similarity in composition, i.e.. they have a certain number of 
species in common. With respect to the grouping of their indi- 
viduals, they are distinguished as open when the plants and plant 
groups are scattered, and dosed when they are so crowded that 




Fig. 90. — The gravel slide a xerophytic formation of the Rocky Mountains. 

invasion is very difficult. Finally, a formation is said to be mixed 
when, owing to position or development, it is really a mixture of 
two or more distinct ones. 

245. Classification by habitats. We have already found that 
the most important differences of habitats so far as plants are 
concerned are due to water and light, What is true of plants 
must hold as well for the formations which they compose. In 
consequence, three great groups of formations are recognized 
corresponding respectively to hydrophytes, mesophytes, and xero- 
phytes. Upon the basis of light, mesophytic formations are sub- 
divided into sun and shade formations. Within each group par- 
ticular formations are arranged according to the type of habitat, 
i.e., pond, forest, meadow, dune, etc. Meadow formations, for 



232 



PLANT PHYSIOLOGY AND ECOLOGY 



example, may differ from each other considerably or entirely in 
the species which compose them, but they are essentially alike 
in character and structure and hence belong to the same type. 

The names of the particular formations of each type are ob- 
tained by adding the name of one or more facies to the general 




Fig. 91. 



-The aspen forest, a mesophytic formation, 
drier ridges. 



Pines appear on the 



name of the type. Thus, an oak-hickory forest is essentially 
different from a balsam-spruce forest, though both belong to the 
group of forest formations. The buffalo-grass prairie is similarly 
distinct from the grama-bluestem prairie, etc. The same formation 
may occur in two or more separate localities, and in this case 
geographical terms are used to distinguish the different examples. 
246. Types of formations. The following list shows the 
arrangement of the more common habitats. 
I. Hydrophytic formations 

(1) pond (5) ditch 

(2) marsh, bog (6) meadow thicket 

(3) stream (7) bank 

(4) spring (8) sandbar 



THE PLANT FORMATION 



233 



II. Mesophytic formations 

1. Shade plant formations 

(9) forest 

(10) grove 

(11) open woodland 

(12) thicket 
III. Xerophytic formations 

(17) desert 
(IS) plains 

(19) prairie 

(20) sanddraw 

(21) strand 



Sun plant formations 

(13) meadow 

(14) pasture 

(15) cultivated field 

(16) wastes 

(22) dune 

(23) cliff 

(24) saline area 

(25) heath 




Fig. 92. — A bog of marsh-marigolds, a hydrophytic formation. 

247. Developmental or physiographic classification. All the 

formations that belong to one succession are classified together 
by virtue of being stages in the development of it. Such a grouping 
is often connected with changes in physiography. The forma- 
tions are* in each case arranged in the sequence found in the par- 
ticular succession. A developmental classification is of great 
importance because it summarizes the history of striking changes 



234 



PLANT PHYSIOLOGY AND ECOLOGY 



in vegetation. The basis is entirely different from that of habitat 
classification and in consequence the two supplement each other. 
Both necessarily deal with the same formations, and are used to 
give different points of view of the vegetation of a region. The 
habitat classification is simpler in that it considers only those 
formations actually on the ground, while development usually 
has to take account of formations that have disappeared. 

248. Regional classification. The grouping together of forma- 
tions that occur in the same region is warranted by the fact that 




Fig. 93. — An open formation of pines. 



they have a larger or smaller number of species and genera in 
common. This has arisen from the mutual invasion constantly 
taking place between adjacent formations. An additional reason 
for such a grouping is furnished by climatic factors, which are 
essentially uniform in each region. Classification with respect to 
general vegetation regions or topographic differences is frequently 
used. Its value lies in furnishing a simple summary of vegetation 
rather than in pointing out an essential relationship. In mountain 
countries, the grouping is a topographic one, determined by 



THE PLANT FORMATION 



l>:;;> 



altitude, as follows: (1) lowland formations, (2) upland forma- 
tions, (3) foothill formations, (4) subalpine formations, (5) alpine 
formations, (6) niveal formations. 

249. Open and closed formations. These terms refer to the 
completeness with which the ground is occupied by plants, and 
indicate the relative ease with which newcomers may invade it. 
In open formations the habitat is slightly or partially occupied, 
and new plants enter readily without displacing those already 
present. The species of closed formations occupy the ground com- 




Fig. 94. — A closed spruce formation. 

pletely. The competition is intense and new plants can enter 
only by displacing some of the original ones. Open formations 
form the earlier stages in the development of a particular area 
of vegetation. They are typical of new' or denuded soils, such as 
blowouts, dunes, flooded areas, gravel slides, burned places, etc. 
Closed formations constitute the later stages of a succession, and 
especially the final or stable condition. Forest, thicket, meadow, 
and prairie are good examples. 

250. Mixed formations. These are produced by the inter- 
mingling of the species of two or more adjacent formations, or of 



236 



PLANT PHYSIOLOGY AND ECOLOGY 



two successive stages of the same succession. The former usually 
results in a zone of varying width between the formations concerned. 
When one stage of a succession is gradually replaced by another, 
the entire habitat is often occupied for many years by a more or 
less equal mixture of the two. It is usually possible to determine 
the formations that produce the mixed one, while the relative 
abundance of their respective species indicates which formation 




Fig. 95. — A mixed formation of aspens and spruces, produced by the com- 
ing in of the spruces, which will finally replace the aspens. 

plays the most important part. Since mixed formations often 
persist for a long time, it is often necessary to consider them in 
detail, very much as though they were distinct formations. In 
some cases it is probable that new formations have arisen by the 
permanent mixing of two contiguous ones. 

Experiment 62. Comparison of formations. Make a general study of 
several formations of the neighborhood. Determine the facies and 
principal species of each, and find out what species they have in common. 
Classify them with respect to habitat, distinguishing open from closed 
ones, and point out any that seem to be mixed formations. 



CHAPTER XII 
AGGREGATION AND MIGRATION 

251. Aggregation. The coming together of individual plants is 
the process that produces vegetation. It gives rise to the innumer- 
able groups of varying rank which taken together make up vegeta- 
tion. This process is aggregation. In its simplest form aggregation 
is the immediate result of reproduction, but as a rule the movement 
or migration of the individuals plays an equally important part. 
The degree and kind of aggregation are consequently determined 
by the relation between reproduction and migration. Aggregation 
is also affected by the abundance of the parent individuals, which 
may occur singly or in groups. 

The simplest cases of aggregation are independent of migration. 
Aggregation gives rise at once to competition between the aggre- 
gated individuals, and upon the outcome of this depends adjust- 
ment or establishment. In the majority of cases simple aggrega- 
tion is prevented by the migration of the seeds or fruits away 
from parent plants. The result, however, is the same, the final 
grouping of the individuals depending upon competition and 
ecesis. 

252. Simple aggregation. The simplest examples of this process 
occur in such algae as Gloeocapsa and Tetraspora, in which the 
plants resulting from fission are held together by a mucilaginous 
substance. The relation between the plants is essentially that of 
parent and offspring, even when the parent disappears regularly 
as in the fission algae, or sooner or later as in the case of annuals. 
Such a group of individuals is a family, and corresponds more or 
less closely to the family in human society. Practically the same 
grouping occurs in the case of terrestrial forms, flowering plants 
especially, when the seeds of a plant mature and fall to the ground 
about it. The size and density of the family group are determined 

237 



238 



PLANT PHYSIOLOGY AND ECOLOGY 



by the manner of reproduction and especially by the number of 
seeds produced. The character of the family is also affected by 
the height and branching of the plant and by the position of the 
seeds upon it. In the case of species whose fruits or seeds are 
immobile, i.e., not well adapted for migration, the seeds fall directly 
beneath the parent. The resulting family is small and definite. 
A similar group is often produced by offshoots when these do not 
carry the new plants too far from the original one. If the fruits 
or seeds are readily carried, i.e., are mobile, the degree of aggre- 




Fig. 96. — Simple aggregation of Corispermum seedlings beneath and about 

the parent plant. 

gation in the family is correspondingly decreased, since the seeds 
are carried away from the parent. Very mobile forms, such as 
the dandelion, rarely produce families for this reason, and this 
is often true also of plants which produce few seeds. Annuals 
occur more frequently in families, owing to the large number of 
seeds and the frequent absence of devices for migration. Many 
perennials also arrange themselves in families. This is true of 
immobile perennials as well as those that migrate by means of 
underground parts. 



AGGREGATION AM) MIGRATION 



2:59 



The above illustrates the law of simple aggregation, viz., that 
lack of dissemination promotes the grouping of parent and offspring 

into families, while mobility hinders it. The production of families, 
moreover, takes place more readily in new and denuded habitats, 
i.e., in open formations, than in closed ones, since in the latter 
the individuals of various species have already become mingled 
with each other. If all species were immobile, families would be 




Fig. 97. — Mixed aggregation due to the migration of Wagnera until the 
family includes individuals of Mentzelia, Elymus, etc., and changes into 
a community. 



characteristic of vegetation to a large degree. Since the great 
majority are more or less mobile, groups of this sort are the ex- 
ception rather than the rule. 

253. Mixed aggregation. Individuals are carried away from the 
family group by migration, and strange individuals are brought 
into it by the same means. In the early growth of a family, due 



240 PLANT PHYSIOLOGY AND ECOLOGY 

to the gradual spreading of the plants, neighboring families ap- 
proach each other and finally mingle more or less completely. 
In both cases the mixing of the two or more species to form a 
community is due to migration. The conversion of a family into 
a community takes place usually through the invasion of mobile 
species. The change occurs when one or more individuals of a 
second species becomes established in the family group. The real 
nature of the community becomes more evident when several 
generations have brought about a considerable increase in number. 
A community is a group of two or more families, regardless of the 
number of individuals in each. The families may remain more 
or less distinct from each other, or may become so intermingled 
that their identity is completely lost. The first condition is 
frequent in open formations, while the latter is the rule in closed 
ones, in which sufficient time has elapsed for repeated migration 
in all directions. 

Communities vary greatly in size and definiteness. They may 
contain but two species, or they may consist of a large number. 
They may be entirely distinct, as often happens in open forma- 
tions when they are separated from each other or from families 
by the bare soil. In denser vegetation they may be fairly dis- 
tinct, or may blend into each other, and finally become indis- 
tinguishable. 

Experiment 63. Study of families and communities. Stake out a 
permanent quadrat to include one or more distinct families, and one to 
include a definite community, preferably in an open formation. Make 
a chart of each quadrat. After the first year the development of family 
and community may be traced by comparing the new charts with those 
of previous years. 

254. Migration. Migration includes all movements by means 
of which plants are carried from the original home, or away from 
the parent individual. It is distinct from ecesis, the act of be- 
coming adjusted or established, but together with it gives rise 
to invasion, which contains the two ideas of movement and estab- 
lishment. An analysis of migration shows that four factors enter 
into it, viz., mobility, agent, distance, and topography. These 
are not all present in each case of migration, but as a rule each 
factor plays some part. Mobility is the capacity of a plant for 
migration, as shown by the various modifications of fruit, seed, 



AGGREGATION AND MIGRATION 241 

etc. The value of mobility to the plant is dependent upon the 
presence of proper agents for causing movement, and the opera- 
tion of these two factors is much affected by distance and 
topography. 

255. Mobility. Mobility indicates the power of the plant for 
movement. It depends primarily upon devices for bringing 
about dissemination, though the number of seeds is also an im- 
portant factor in it. Mobility is most marked in those plants 
which are themselves motile, bacteria, diatoms, vol vox, etc. r 
or possess motile spores, such as most of the green algae. On the 
other hand, it is little or not at all developed in those flowering 
plants with large, heavy seeds or fruits. By far the greater number 
of plants exhibit some degree of mobility. The range is extreme, 
from the almost immobile offshoots of lilies, which move by growth, 
to the non-motile but very mobile spores of fungi. There is no 
necessary correspondence between mobility and motility. The 
latter is practically absent in terrestrial plants, and, in spite of 
its importance among the algae, it plays a relatively small part 
in migration. The degree of mobility is determined chiefly by 
the nature of the device used in dissemination, but the number of 
seeds or spores produced has an important effect in increasing or 
decreasing it. A third factor of much influence is the position 
of seed or spore with reference to the action of the distributive 
agent. 

256. Organs of dissemination. Plants differ much with respect 
to the organ modified or utilized for dissemination. Such modi- 
fication, while it usually affects the fruit or seed alone, may act 
upon any organ, or upon the entire plant body. Special modifica- 
tions are usually developed in connection with spores and seeds, 
and mobility is most marked in species of this sort. It is much 
reduced in the case of offshoots and plant bodies, at least in terres- 
trial plants, notwithstanding a few striking exceptions, such as 
the tumble-weeds. The following indicates the grouping of plants 
with reference to the part distributed. 

1. Spore-distributed. This group includes all plants possessing 
structures which are called spores, viz., algae, fungi, liverworts, 
mosses, and ferns. Spores rarely have special devices for dis- 
semination, but their minute size makes them extremely mobile. 

2. Seed-distributed. This group comprises all flowering plants" 
in which the seed is the part modified or disseminated. Seeds are; 



242 PLANT PHYSIOLOGY AND ECOLQGY 

not very mobile, except when they are minute, or are provided 
with wings or hairs. 

3. Fruit-distributed. The modifications of the fruit for dis- 
tribution exceed in number and variety all other modifications 
for this purpose. Many structures, such as the achene, caryopsis, 
perigynium, utricle, etc., which are commonly mistaken for seeds, 
belong here. 

4. Offshoot-distributed. To this group are referred all plants 
that produce lateral shoots, such as root-sprouts, rhizomes, runners, 
stolons, etc. The migration of such plants is very slow, but it is 
unusually effective, since the new plant is nourished by the parent 
until it becomes fully established. 

5. Plant-distributed. This group includes submerged and 
surface water plants, both motile and non-motile, and those land 
forms in which the whole plant, or at least the aerial part, is dis- 
tributed, as in tumble-weeds and, many grasses. 

257. Modifications for migration. Plants are arranged in the 
following groups according to the nature of the device by which 
migration is brought about. 

1. Saccate (saccospores). The species of this group possess 
various fruits all of which agree in having a sack-like envelope. 
This may be membranous and serve for wind-distribution, as in 
Ostrya, Physalis, and Staphylea, or impervious and air-containing, 
as in Carex, Nymphcea, etc., where it serves for water-transport. 

2. Winged (pterospores). This group includes all winged, 
margined, or flattened fruits and seeds, such as are found in Acer, 
Betula, Rumex, many Umbelliferce, Graminacece, etc. 

3. Comate (comospores). To this group belong those fruits 
and seeds with long silky hairs, Anemone, Asclepias, Gossypium, 
etc., and those with straight capillary hairs or bristles not confined 
to one end, Salix, Typha, etc. 

4. Parachute (petasospores). These are the parachute-like 
achenes of Lactuca, Taraxacum, and other ligulate composites. 
Through Eriophorum, Senecio, etc., this group is connected with 
the preceding one. Parachute fruits represent the highest degree 
of mobility that has been obtained by special modification. 

5. Chaffy (carphospores). In this group are placed those 
achenes with a more or less scaly or chaffy pappus which gives 
slight mobility, as in Brauneria, Helianthus, etc. 

6. Plumed (lophospores). In fruits of this kind the style is 



AGGREGATION AND MIGRATION 243 

the pari usually modified into a long, plume-like organ, producing 
a high degree of mobility, as in Clematis, Pulsatilla, and Sieversia. 

7. Awned (acospores). These are nearly all grasses, in which 
the awns serve for distribution by wind, water, or animals, and 
even by certain creeping movements. The degree of mobility 
in many cases is great. 

S. Spiny (centrospores). This group contains a few species in 
which distribution of the spiny fruits is brought about by attach- 
ment, as Cenchrus and Tribulus. The mobility is fairly high. 

9. Hooked (oncospores). The members of this group are ex- 
tremely numerous, and the degree of mobility as a rule is very 
high. All agree in the possession of hooks and barbs, which serve 
for attachment, though the number, size, and position of the 
hooks vary greatly. 

10. Viscid (gloeospores). In these the inflorescence is more or 
less covered with a viscid substance, as in species of Silene, or the 
fruit is beset with sticky hairs, as in Cerastium, Salvia, etc. 

11. Fleshy (sarcospores). These are fleshy fruits which are 
scattered in consequence of being swallowed, especially by birds. 
The seeds are usually protected by a stony envelope which enables 
them to resist digestion. The mobility varies greatly, but the 
area over which migration may be effected is large. 

12. Flagellate (mastigospores). These are plants with ciliate 
or flagellate spores, as in (Edogonium, Ulothrix, Vaucheria, etc., 
or with plant bodies similarly motile, Bacteriacece, and Volvocaceaz. 

258. Influence of seed production. The chances of migration 
depend in a large degree upon the number of fruits, seeds, or spores 
produced. A large seed production increases the movement of a 
mobile species. In the case of tw r o species w r ith equally good 
devices for distribution, the one with the largest number of seeds 
is the more mobile. Even in immobile plants, seed production 
increases the few chances of movement. 

Two kinds of seed production are distinguished upon the basis 
of the relation between number of seeds and of flowers. In one 
species the flowers are many, but the seeds few or single in each, 
as in composites, grasses, sedges, etc. In the other the number 
of seeds in each flower is large, as in lilies, orchids, violets, etc. 
In so far as the actual number of seeds produced is concerned, 
some species of one type do not differ greatly from some of the 
other. As a rule, however, species with many flowers are more 



244 PLANT PHYSIOLOGY AND ECOLOGY 

highly specialized for migration, and are consequently more mobile. 
The number of fertile seeds is also much greater, a fact of much 
significance, since the movement of abortive seeds is of no benefit 
to the species. This fact taken in connection with their great 
mobility partly explains the supremacy of composites and grasses- 

259. Position of disseminules. The position on the plant of 
the part disseminated, i.e., its exposure to the distributing agent, 
plays a part in mobility. In the majority of flowering plants, the 
position of the inflorescence gives a maximum of exposure, but 
in many cases special modifications are developed to place spores 
or seeds in a more exposed position. The height of the inflorescence 
from the ground or above the surrounding plants aids in increasing 
the distance to which the spores or seeds are carried in the first 
flight. 

The most perfect device of this kind is found in such com- 
posites as the dandelion, in which the stalk stretches up after 
the head closes finally. By the time the involucre expands to 
release the parachute fruits, the flower stalk has grown to several 
times its original length. The carpotropic movements of various 
plants often serve to place seeds and fruits in a better position 
for dissemination. In certain composites the involucral scales 
are reflexed at maturity, thus loosening and lifting up the achenes. 
A somewhat similar result is obtained in such grasses as Stipa 
and Aristida by the twisting of the awns. In many mosses, liver- 
worts, and puffballs, the spores are sifted out through slits or teeth, 
or the whole spore mass is elevated and held apart by the mass 
of elaters or threads. In most cup-fungi the spores are driven 
out of the cup by tensions within. 

260. The agents of migration. The possibility of migration 
depends primarily upon the action of distributing agents. In the 
absence of these even the most perfect modification is without 
value, while their presence often brings about the movement of 
the most immobile plant. The amount and extent of migration 
are determined chiefly by the permanence and forcefulness of the 
agent concerned. Furthermore, the direction and rapidity of 
migration depend upon the direction and intensity of the agent. 

Migration results when spores, seeds, fruits, offshoots, or plants 
are moved out of their homes by water, wind, animals, man, 
gravity, glaciers, growth, or mechanical propulsion. In accord- 
ance the following groups are distinguished: 



AGGREGATION AM) MIGRATION 



24. 



1. Water (hydrochores). This group comprises all plants dis- 
tributed by water whether id the form of ocean currents, tides, 
streams, or surface run-off. In the case of streams and run-off 
especially, the nature of the modification is of little importance, 
provided the disseminules are impervious, or little subject to injury 
by water. Motile plants, or those with motile cells, belong entirely 
to this group. 

2. Wind (anemochores). The group of wind-distributed species 
includes practically all terrestrial plants in which modifications 




Fig. 98. — Migration of the immobile seeds of Corispermum by means of 

surface drainage. 

for increasing the surface of seed or fruit have been greatly developed, 
or in which the part carried is minute. Sack-like, winged, hairy, 
parachute, pappose, plumed, and some awned seeds or fruits 
are the various types of modification for wind-distribution. 

3. Animals (zoochores). Animals distribute seeds in conse- 
quence of attachment, carriage, or use as food. Dissemination 
by attachment has been specialized in a high degree. The three 
types of contrivances for this purpose are found in spinose, hooked, 
and glandular fruits. Distribution by swallowing and that by car- 
riage often play a striking part on account of the great distance 



246 



PLANT PHYSIOLOGY AND ECOLOGY 



to which the seeds may be carried. The one is characteristic of 
fleshy fruits, and the other of nut fruits. 

4. Man (brotochores) . Distribution by man has no necessary 
connection with mobility. It acts through great distances and over 
immense areas, as well as near at hand. It may be intentional, as 
in the case of cultivated plants, or unintentional, as in thousands 
of native or foreign species. No other disseminating agent can 




Fig. 99. — A plant of an aster established near the top of a tall chimney 
through the agency of the wind. 



compare with man in respect to the amount or distance of migra- 
tion. 

5. Gravity (clitochores) . Gravity is an agent of migration in 
hilly and mountain regions, where seeds and fruits regularly reach 
lower positions, either by falling from cliff or rock, or, more fre- 
quently, by the breaking away and rolling down of rock or soil 
masses. Dissemination by this method is necessarily local, though 



AGGREGATION AND MIGRATION 247 

it plays an important part in the rock fields and gravel slides of 
mountains, especially in the case of immobile species. 

6. Glaciers (crystallochores). Transport by glaciers is of slight 
importance at the present time, because of its restriction to alpine 
and polar regions, where the flora is poorly developed. In con- 
sidering the migrations of the glacial epoch, however, distribution 
by glaciers is an important factor. 

7. Growth (blastochores). The mobility of species dissemi- 
nated by the growth of offshoots is extremely slight, and the annual 
movement relatively insignificant. The certainty of migration 
and of ecesis is so great, however, and the presence of offshoots 
so frequent in terrestrial plants that growth plays an important 
part in migration, especially within formations. 

8. Propulsion (bolochores). Dissemination by mechanical pro- 
pulsion, though it operates through insignificant distances, is very 
important on account of its cumulative action from year to year. 
The number of plants with contrivances for propulsion is very 
much smaller than the number of those with offshoots. All species 
of this group agree in having modifications by w T hich a tension is 
established. At maturity this tension suddenly overcomes the re- 
sistance of sporangium or fruit, and throws the enclosed spores 
or seeds to some distance from the parent plant. In accordance 
with the manner in w r hich the tension is produced, sling-fruits are 
classified as follows: 

(a) Hygroscopic fruits. These include the ferns with annulate 
sporangia in which the expansion of the annulus by the absorption 
of moisture bursts the sporangium more or less suddenly. The 
actual propulsion of the spores seems to be caused by the reflex 
movement due to drying. 

(b) Turgescent fruits. Propulsion by turgescence occurs in a 
large number of fungi, such as the fleshy Discomycetes , etc. Among 
flowering plants, Impatiens and Oxalis are familiar examples of 
fruits which split in consequence of increased turgidity. 

(c) Dry fruits. The number of fruits which dehisce upon 
drying is very large, but only a small portion of these expel their 
seeds forcibly. Erysimum, Lotus, Viola, and Geranium illustrate 
the different ways in w T hich drying brings about the sudden splitting 
of fruits. 

(d) Mortar fruits. In some plants, especially composites, 
borages and mints, the achenes or nutlets are so placed in the 



248 PLANT PHYSIOLOGY AND ECOLOGY 

persistent involucre or calyx that the latter serves as a kind of 
mortar for the projection of the seeds when the stem is sharply 
bent to one side by any force such as the wind or some animal. 

261. The work of migration agents. Two or more agents some- 
times act upon the same disseminule, usually in succession. The 
possibility of such action in nature is great, but actual instances 
of it are not frequent, except when the activities of man enter 
into the question. Some parts, such as awned inflorescences, are 
carried almost equally well by wind or animals, and are often 
scattered by the action of both. Seeds and fruits are frequently 
blown by the wind into streams by which they are carried away. 
As a rule, however, parts adapted to wind-distribution are injured 
by immersion in the water, and the number of plants capable of 
being scattered by the successive action of wind and water is 
small. As a general rule, plants growing in or near the water, 
if modified for migration at all, are adapted to water-carriage. 
Species that grow in exposed grassy or barren habitats are for 
the most part wind-carried. Those found in the shelter of forests 
and thickets are usually scattered by animals, though the taller 
trees and shrubs are generally wind-distributed by reason of 
exposure to the upper air-currents. There is seen to be a certain 
amount of correspondence, since hydrophytes are usually water- 
carried, shade plants are borne by animals, and the majority of 
sun mesophytes and xerophytes are wind-distributed. In each 
group, however, are numerous exceptions to the rule, owing to 
migration into various types of habitats. 

With respect to their action, agents are constant or intermittent. 
The former include currents, streams, winds, gravity, growth, 
and propulsion; the latter, animals, including man. In the case 
of constant agents migration takes place more or less continuously 
from year to year, and usually in a definite direction. With 
intermittent agents dissemination is largely accidental; it is 
indeterminate in direction, and recurs only at irregular intervals, 
if at all. Migration is most effective when it is continuous, and 
least when it is intermittent. In the one case the migration is an 
annual one with the probability of the gradual adjustment of the 
seedling. In the other, species are usually carried not only out 
of their particular habitat, but often far beyond their native 
region, and establishment may become difficult or impossible. 
The rapidity of migration is usually greatest for intermittent 



AGGREGATION AND MIGRATION 249 

agents, though it varies much for the same agent. The distance 
of migration is variable. It is often greatest in the case of man, 
other animals, ocean-currents, and wind, and small or scarcely 
perceptible when the movement is due to gravity, growth, or 
propulsion. Seals may be carried half-way across a continent in 
a week by strong-flying birds, while migration by growth or expul- 
sion is limited to a few centimeters or at most a few meters per 
year. This slowness is partly counterbalanced by the greater 
number of disseminules, and the much greater chance of becoming 
established. 

Experiment 64. Modifications for migration. List a number of 
species of the flora according to formations and arrange them in a table. 
Divide the table into five columns, and record the behavior of each 
species with respect to the part modified, the kind of modification, the 
seed production, position of disseminule, and the agent of migration. 

262. The direction of migration. The direction in which a 
migrant moves is determined by the agent concerned. The general 
movement is forward or outward, the lines of travel radiating in 
all directions from the parent area. This is well illustrated by 
the action of winds which blow from any quarter. In the case of 
constant winds, migration is more or less definite, the exact direc- 
tion being determined largely by the fruiting period of the species 
concerned. The position of invaders with reference to the original 
home does not necessarily indicate the only direction of migration, 
since seeds are regularly carried to places in which they can not 
obtain a foothold. 

The local movement of plants carried by animals takes place 
in all directions, while their distant migration follows the path- 
ways of the migratory birds or mammals. Distribution by man 
is determinate when it takes place along commercial routes or 
along highways. In ponds, lakes, and other bodies of standing 
water, migration usually occurs in all directions, but in ocean- 
currents and streams it is determinate except for motile species. 
Dissemination by gravity, slopes, and glaciers is local and definite, 
while propulsion is entirely indeterminate. Migration by growth 
is equally indefinite, but it produces a radiate movement away 
from the parent mass, while propulsion throws seeds into the 
mass as readily as away from it. From the preceding it is evident 
that distant migration may take place by means of water, wind, 
animals, or man, and that it is in some degree determinate, since 



250 PLANT PHYSIOLOGY AND ECOLOGY 

these agents usually act in a definite direction over great distances. 
On the contrary, local migration is indeterminate as a rule, except 
in the case of streams, glaciers, and slopes. The direction of 




Fig. 100.— Determinate migration of the blue columbine, 
Aquilegia ccerulea, down a gravel slide. 

migration is thus seen to be controlled by the distributive agent. 
The distance is determined by the intensity and duration of the 
agent, as well as by the nature of the area through which it acts. 

Experiment 65. Amount and direction of migration. Make a general 
study of a railroad track for the migration of introduced plants, espe- 
cially weeds, and of a stream which flows through a prairie or meadow 
for the movement of forest species. 

Establish two migration circles. Select a plant or group with 
comate or parachute fruits as the base for one, and an immobile species 
for the other. If the plant chosen is just beginning to loosen its fruits, 
the use of a circle of 2 m. radius shows the direction of movement 
and the varying distance of the first flight. The bases of the two circles 
are made permanent by driving a stake. In late spring the circles 
are again examined, the new plants counted, and their relative position 
with respect to the parent plant recorded. 



CHAPTER XIII 
COMPETITION AND ECESIS 

263. Competition. In consequence of aggregation, two or 
more individuals come to occupy the space previously occupied 
by one, or the group of plants already in possession of an area is 
greatly increased in number. Usually the immediate result is 
competition between the various individuals, though this is not 
always the case. The number of individuals may be so small 
and the distance between them sufficiently large, so that they do 
not compete with each other. Actual competition begins only 
when one plant encroaches upon the space occupied by another. 
Moreover, it sometimes happens that species are so different in 
their nature and their demands upon the habitat that they may 
be crowded together without being in actual competition. 

Competition occurs only between plants that meet each other 
on terms more or less equal. It is impossible to speak of competi- 
tion between an oak and the tiny herb that grows beneath it, or 
between a puffball and the prairie grasses which surround it. 
Likewise, there is no competition between a host-plant and the 
parasite upon it, though two or more parasites upon the same 
host may compete with each other. Parasite competes with 
parasite and host with host, though a rust, for example, may 
often be a decisive factor in the competition between two wheat 
plants. 

264. The struggle for exiltence. This popular phrase contains 
two different ideas. As Darwin pointed out, a plant may struggle 
against adverse conditions in the habitat, or it may struggle with 
other plants for things necessary to it. A pioneer migrating into 
a new or denuded habitat must for a time make its way against 
conditions more or less unfavorable to it. Establishment, or 
ecesis, can take place only when it succeeds in adjusting itself. 

251 



252 PLANT PHYSIOLOGY AND ECOLOGY 

When a plant is carried into a group of other plants, or is sur- 
rounded by its growing offspring, the struggle which results between 
the individuals is competition. In all cases where a migrant is 
carried into the vegetation of a very different habitat, it must 
meet both tests. In consequence, ecesis usually includes adjust- 
ment to competition as well as to physical factors. Competing 
plants are really trying to obtain certain necessary amounts of 
physical factors. Properly speaking, the struggle for existence 




Fig. 101. — Competition in a family of Corispermum seedlings. The com- 
petition is much keener between the seedlings that have sprung up 
densely beneath the dead parent plant than between those arising 
from the seeds scattered between the parents. 

in the plant world is a struggle between each plant and its habitat, 
the latter being changed by competition in consequence of the 
demands made upon it by other plants. The only exceptions to 
this rule are furnished by plants which serve as hosts to parasites. 
Between host and parasite there is a struggle not very different 
from that between two animals. 

265. The nature of competition. Competition is purely a 
physical process. With a few exceptions, such as the crowding 



COMPETITION AND ECESIS 253 

up of tuberous plants when grown (oo closely, an actual struggle 
between competing plants never occurs. Competition arises 
from the reaction of one plant upon the physical factors about it, 
and the effect of those modified factors upon its competitors. In 
the exact sense, two plants, no matter how close, do not compete 
with each other as long as the water content and the nutrient 
material, the heat and light, are in excess of the needs of both. 
When the immediate supply of a single necessary factor falls below 
the combined demands of the plants, competition begins. 

266. The factors involved. All of the factors essential to 
one or more of the primary functions of the plant play some part 
in competition. Such factors are water, humidity, light, and 
temperature. Of these humidity and temperature are relatively 
unimportant, while water content and light are decisive. In 
wet soils the question of air content seems to be of importance, 
but ordinarily this is apparently not true. 

Plants are sometimes said to compete for room. This view 
is incomplete, and probably has resulted from the fact that plants 
show the effects of competition the more the closer they grow. 
The explanation is that the amount of water and light available 
for each decreases as the plants become more crowded. The 
moment that the roots of one enter the area from which the other 
draws its w r ater supply, or the foliage of one overshades the leaves 
of the other, a change in factor results, which is unfavorable to 
one or the other, and competition begins. 

267. Competition for water and light. Plants that grow in 
close or crowded masses compete with each other for water or light. 
In the majority of cases both factors are involved. Plants with 
a larger, deeper, or more active root system react upon the habitat 
and reduce the amount of water available for those with poorer 
root systems. The stronger, taller, more branched or more leafy 
plants receive the larger share of the sunlight, and the others can 
obtain only what is left. This action of one competitor upon 
the habitat, and of the habitat upon the other competitor is cumu- 
lative. An increase in the leaf surface of a plant not only reduces 
the amount of light available for the plant near it or beneath it; 
it also renders necessary the absorption of more water and nutrient 
salts, and correspondingly decreases the amount available. The 
result is that the successful individual prospers more and more, 
while the less successful one loses ground in the same degree. As 



254 PLANT PHYSIOLOGY AND ECOLOGY 

a consequence, the unsuccessful competitor disappears entirely, 
or is so handicapped that it produces fewer or less vigorous seeds. 

Competition for both water and light is the rule when plants 
of varying height are intermingled. This is always true when the 
plants are broad-leaved or branched. In the case of grasses, 
many mats, rosettes, etc., competition for light is relatively unim- 
portant on account of the size or position of the leaves. With 
such forms as the leafless sedges and rushes, it seems to be entirely 
lacking. Plants which grow in saturated soils or in water appar- 
ently do not compete for the latter, though it is probable that 
a new factor, air content, enters the question. Finally, it is 
possible for plants to be densely crowded, and still not compete 
with each other This is nicely illustrated by duckweeds which 
often completely cover the surface of ponds and streams. The 
tiny fronds are on an equality with respect to light, and the water 
supply is far in excess of the demand. 

268. Competition between parents and offspring. The simplest 
kind of ordinary competition is that in which the individuals 
belong to the same species. The various individuals of a family 
show relatively slight differences in height, width, leaf expanse, 
and root surface. Some have surfaces which are larger or better 
situated for receiving water or light, and the others are thus placed 
at a disadvantage. The former receive more than their share 
of water or light, or of both. The reaction which they produce 
upon these factors affects the plants subject to it. The usual 
result of such competition is great variation in the height, branch- 
ing, and leaf area of the different individuals, and the inability 
of many to produce flowers. This is particularly true of annuals 
and of perennials belonging to the same generation. In the 
competition between the parents and offspring of the same per- 
ennial species, the former usually have a decided advantage. 
The younger plants are often unable to thrive, or even to germinate, 
and they finally disappear, leaving a free space beneath and about 
the stronger parents. Similar individuals make practically the 
same demands upon the habitat, and adjust themselves least 
readily to their mutual reactions. The more unlike plants are 
the greater the difference in their needs. Hence some are able to 
adjust themselves to the reaction of others with little or no dis- 
advantage. From this is obtained the primary law of competition, 
i.e., competition is closest when the individuals are most similar. 



( OMPETTTION AND Erf ESIS 255 

Consequently competition is more intense within families than 
within communitii 3. 

269. Competition between different species. Competition is 
closer between species oi like form than between those that are 
dissimilar. Such similarity between species is based upon the 
form or nature of the plant body, and not upon systematic relation- 
ship. Leaf, stem and root characters ordinarily determine the 
outcome. The species most alike in these respects will be in 
close competition, regardless of taxonomic relationship. This is 




Fig. 102. — Competition between different species, in this case mountain 
daisies and grasses. 

equally true of species of the same genus, and of those belonging 
to genera of widely separated families. This relation is expressed 
by a second law of competition, viz.. the closeness of the competition 
between individuals of different species varies with their similarity 
in vegetation form or habitat form. 

This law applies especially to the competition which arises 
between occupants and invaders in the various stages of succes- 
sion. Those invading species that show the greatest resemblance 
to occupants in the form of leaf, stem, or root experience the 
greatest difficulty in establishing themselves. On the contrary, 



256 PLANT PHYSIOLOGY AND ECOLOGY 

species which are so unlike the occupants that they enter at a 
clear advantage or disadvantage usually establish themselves 
readily. This principle lies at the base of the changes in suc- 
cession which give a peculiar stamp to each stage or formation. 
A reaction sufficient to bring about the disappearance of one 
stage can be produced only by the entrance of invaders so differ- 
ent in form or nature that they change the impress of the for- 
mation materially or entirely. A formation becomes stable 
when the entrance of such invaders is no longer possible. For 
example, while many vegetation forms can still enter a forest, 
none of these are able to place the trees at a disadvantage. As 
a consequence the final forest stage, though it may change in 
composition, can not be displaced by another. 

270. Influence of vegetation form and habitat form. The 
course and the result of competition are primarily dependent 
upon the vegetation form and the habitat form of the competing 
species. Species of the same vegetation form, i.e., two or more 
kinds of shrubs, compete closely with each other, and the result 
is a reduction in the number or size of the individuals, or the 
entire disappearance of one or more species. On the other hand, 
dissimilarity in vegetation form tends to diminish competition 
and to maintain the advantage of the superior form. Species 
of trees, as a rule, compete sharply with each other when found 
together. The same is true of other vegetation forms, shrubs, 
rosettes, etc. The relation of the shrubs to the trees or of the 
rosettes to the shrubs of a formation is one of subordination and 
not of competition. The matter of height and width often plays 
an important part in deciding this question. The amount and 
disposition of leaf surface are decisive factors in competition 
between species of the same vegetation form, in so far as this is 
governed by light. In plants in which the leaves are usually 
erect, e.g., grasses and sedges, competition between aerial parts 
is slight, and the result is chiefly determined by the roots. The 
effect of a difference in habitat form is unusually marked, since 
the invader must then adjust itself to the more or less unfavor- 
able conditions of a new habitat, in addition to meeting the test 
of competition. 

271. The effect of position. The position of the competing 
individuals is of the greatest importance, as already indicated. 
The distance between the plants directly affects the degree of 



COMPETITION AM) ECESIS 257 

competition, i.e., the latter increases as the distance diminishes 
and the reverse. Their arrangement, i.e., whet her in families 

or communities, and whether the individuals are scattered or 
grouped by species, exerts a marked influence by determining 

that the contest shall be between like forms or unlike forms. 
Position is controlled primarily by seed production and dissem- 
ination, though it is influenced in a large degree by the location 
of the first points occupied by an invader. The individuals of 
species which produce many seeds and are relatively immobile usually 
occur in dense stands. In these the competition is intense for 
the two reasons of similarity of form and density of arrangement. 
As a result the plants fall below the normal in height and width. 
When the seed production is small or the mobility great, the 
individuals are scattered among those of other species, and the 
closeness of the competition depends largely upon the similarity 
existing between them. 

272. Vegetation pressure. Masses of vegetation are often 
said to force the weaker plants or species toward the edge, thus 
producing an outward or forward pressure. Plants are likewise 
said to have been driven into the water or into unfavorable habitats 
by the pressure of stronger ones. The movement concerned is 
merely migration, which may or may not be followed by ecesis. 
It is determined by the nature of seed or fruit, and has no con- 
nection with stronger or weaker species, or the presence of a vital 
pressure. The direction taken by the migrants is largely inde- 
terminate. Migration is outward, or away from the mass, because 
it is radiate. The chances of ecesis are greatest at the edge, where 
the similarity between the plant forms is less, and the competition 
correspondingly diminished. Hence the actual movement or 
invasion is outward. 

In the same way, the question of water plants is merely one 
of migration and ecesis. The plants that grow at the edge of a 
pond or stream are not forced into the water by stronger neighbors. 
They can become water plants only in one of two ways, both of 
which are equally possible for strong and weak species. Their 
seeds may be carried into water, either at one flight, or by a gradual 
movement year by year, and those that adjust themselves become 
water plants. On the other hand, a gradual increase in the water 
of a habitat to the maximum may result in the extinction of some 
forms, and the adjustment of others. 



258 



PLANT PHYSIOLOGY AND ECOLOGY 



273. The results of competition. A plant adjusts itself in 
so far as possible to the changes in factors brought about by its 
competitors. Hence competition may bring about changes in 







Fig. 103. — Effects of competition for light between individuals of Solidago 
canadensis. The tall plants grew in the edge of the family and shaded 
the small plants of the center. 



size, form, and structure of the individuals, or in their arrange- 
ment, i.e., in vegetation. Change in size and general form is 
one of the most striking effects wherever the plants are densely 
crowded. Changes in the form and structure of leaf and root 



COMPETITION AM) ECESIS 259 

occur regularly, and if the conditions which cause them con- 
tinue, they may become fixed. Competition, like the physical 
factors through which it acts, may produce adaptations which 
characterize new forms. 

Competition enters into practically all the changes of vegeta- 
tion. It is absent only in the very beginning of open formations 
on new or denuded soils. It is one of the fundamental factors 
in invasion, and the reaction of competing occupant and invader 
is the decisive element in all successions. Competition plays a 
part in both alternation and zonation, but it is especially character- 
istic of the former. It not only modifies the alternation arising 
from physical factors, but it also exerts strong control over that 
due to the historical factor, i.e.. the time of invasion. 

274. The study of competition. The course of competition 
under natural conditions may be followed in the field, or com- 
petition may be artificially produced and studied in the plant- 
house. There is no essential difference in the nature of the com- 
petition in the two places, but the plant-house is ordinarily more 
favorable because the study can be kept under control. Various 
competition cultures are distinguished with respect to the point 
of attack, 1 but only two of these are used for elementary work. 
Simple cultures are those in which a single species is used. The 
resulting group is a family, and the competition is between like 
individuals. In such cultures the problem of the factors in com- 
petition is reduced to its simplest terms. Mixed cultures are 
composed of two or more species, and are correspondingly more 
complex. Either culture is made permanent by allowing the 
plants to ripen and drop their seeds from generation to gener- 
ation, just as in nature. 

275. Competition cultures are a meter square, i.e., they are 
indoor quadrats. For a simple culture, the plot is usually divided 
into four squares to give different degrees of density. The density 
may be varied as desired, though, as a rule, 12 seeds are planted 
in one, 25 in another, 50 in the third, and 100 in the fourth. In 
making a mixed culture, the problem is simplified by using two 
species only. In one half of the plot the number of seeds of the 
one is double that of the other; in the other half, conditions are 
reversed. At the time the plots are sown, seeds are started in 

1 Research Methods, 310. 



260 



PLANT PHYSIOLOGY AND ECOLOGY 



pots so that they may be grown without competition to serve 
as checks. When the plants become well grown and in actual 
competition, light readings are made for two or three leaves of 
the most successful competitors and of the least successful ones 
and their starch content is also determined. From these figures 
the light intensity and starch content are determined for the 
total leaf area of each plant, and these results are checked by the 
control plants. 

The relation of water to competing plants is a more difficult 




Fig. 104. — Competition cultures in the plant-house. In the foreground 
is a mixed culture of Solidago and Onagra. 



task to study. It is necessary to determine the water available 
for the control, plant, together with its water loss. These must 
also be determined for the competing individuals, and the two 
results compared. The water content is found for a block of 
the soil 2 or 3 decimeters square. The water loss of the plants 
in it can only be ascertained by cutting the block out and weigh- 
ing it from time to time, or by sinking a pot or box in the plot 
at the beginning, and removing it when necessary. 



. COMPETITION AM) ECESIS 261 

276. Competition quadrats. Practically any permanent quad- 
rat will serve for the field study of competition. Denuded quad- 
rats are especially good, though t hey need to be followed for a 
number of years. When rapid results are sought, a denuded 
quadrat is used for a competition culture by planting it in the 
manner desired. Other quadrats are used in a somewhat similar 
fashion by sowing the seeds of a weed or other vigorously grow- 
ing species among the original occupants. Interesting results are 
obtained with annuals by using a decimeter quadrat. Two quad- 
rats of this size are staked side by side in a family of seedlings, 
and the number of individuals counted for one of them. The 
plants are then removed from one. taking care to leave two or 
three scattered ones undisturbed. As they develop, the behavior 
of the plants in the two quadrats indicates clearly the effect of 
competition. 

Experiment 66. Competition. Make a simple and a mixed culture in 
the manner indicated, using Helianthus, Datura, or other vigorous 
plants. Follow the development of each culture carefully, noting the 
differences of form and structure, and determining the relation to light, 
and also to water if this is possible. Allow the plants to go to seed, 
and determine the largest and smallest seed production for those that 
flower. Permit one example of each culture to become permanent for 
study during the following year. 

Stake out two decimeter quadrats in the family of an annual, such 
as Polygonum aviculare, just after the seedlings appear above the 
ground. Count one quadrat and thin out the other. Compare the 
development in the two throughout the season. Note the size and 
number of mature plants in each, and the number of seeds produced. 

277. Ecesis. Ecesis is the adjustment of a plant to a new 
place or a new habitat. It includes all the phenomena shown 
by a migrant from the time it enters a new situation until it be- 
comes thoroughly established there. It is the decisive factor in 
invasion, since migration is of no value unless followed by it. 
The interaction of the two is intimate. Migration is usually fol- 
lowed by ecesis, which then establishes a new center from which 
further migration is possible, and so on. 

Ecesis depends in a large measure upon the time, direction, 
rapidity, distance, and amount of migration. The time of year 
in which fruits mature and distributive agents act has a marked 
influence upon the establishment of a species. Seeds which 






262 



PLANT PHYSIOLOGY AND ECOLOGY 



ordinarily pass through a resting period are often brought into 
conditions where they germinate at once, and perish because of 
unfavorable factors, or because competing species are too far 
advanced. Spores and seeds capable of immediate germination 
may likewise be scattered abroad at a time when conditions make 
growth impossible. The direction of movement is often decisive 
because the seed or spore is either carried into a habitat suffi- 
ciently like that of the parent to secure establishment, or into 
one so dissimilar that germination is impossible, or at least is not 









?^**v 






g^gff* 1 ^:, • 


V~ ■ '*- 


















' 


- 








- ■ 






•; " ■' 


. 










3pjkiI.'0B!^f ' \fflqEtyj"i 


SWj 


^ 










» 



Fig. 105. — Detail of a salt basin formation, showing the ecesis beneath 
and about the parent plants of Corispermum. 

followed by growth and reproduction. The rapidity and dis- 
tance of migration have little influence, except in the case of 
conidia, gemmae, etc., with little resistance to dryness. The 
amount of migration, i.e., the number of migrants, is of the great- 
est importance, since it directly affects the chances that vigorous 
seeds will be carried into places where ecesis is impossible. 

278. The factors in ecesis. Ecesis may take place in a habitat 
covered with plants, or in one that is without a plant covering. 
The first case is by far the most frequent. In it, the invaders 
must not only adjust themselves to the general physical factors 



COMPETITION AM) ECESIS 263 

of the habitat, but also to the changes id them duo to compe- 
tition. Competition is consequently a most important factor in 
the ecesis of an invader in a formation. It is necessarily absent 
when an invader outers a new or denuded soil alone, but it appears 

quickly when a large number of plants invade at the same time. 

Normally, ecesis consists of three essential processes, viz., 
germination, growth, and reproduction. This is the rule among 
terrestrial plants, in which migration regularly takes place by 
means of a resting part. In free aquatic forms the growing plant 
or part is usually disseminated, and ecesis consists merely in 
being able to continue to grow and reproduce. Moreover, ecesis 
is practically certain on account of the slight differences between 
aquatic habitats. In dissemination by offshoots, the conditions 
are somewhat similar, and ecesis consists of growth and repro- 
duction alone, since the offshoot grows under the same condi- 
tions as the parent plant. 

With respect to ecesis, migrants may behave in any one of 
four ways: (1) they may invade an area without germinating; 
(2) they may germinate and then disappear; (3) they may ger- 
minate and grow without reproducing; (4) they may reproduce 
either by flowers or offshoots, or both. Ecesis, and hence in- 
vasion, occurs only when a migrant enters a new place, in which 
it germinates, grows, and reproduces. 

279. Germination of the seed. The germination of seed or 
spore is dependent upon its vitality and upon the nature of the 
habitat. Vitality, or viability, is determined by the structural 
characters of the fruit, seed-coat, and endosperm, as well as by 
the nature of the embryo. The first three promote germination 
by protecting the embryo against dryness or against injury due 
to carriage by water or by swallowing. This duty is chiefly 
discharged by the seed-coat , which usually contains thick-walled 
protective cells. The seed-coat also helps to insure germination 
at the proper time, by making the seed more or less resistant to 
a quantity of moisture and warmth insufficient to support the 
seedling. The presence of the endosperm or other food supply 
increases the chances of the seedling. 

The behavior of the seed with respect to germination depends 
in a large degree upon the nature of the embryo. Many seeds 
are not viable because fertilization has not taken place and the 
embryo has not developed. This explains the low 7 germinating 



264 



PLANT PHYSIOLOGY AND ECOLOGY 



power of some species which produce a large number of seeds. 
The seeds of some plants grow immediately after ripening, while 
others grow only after a resting period of uncertain duration. 
Even in the case of seeds from the same parent, the majority 
germinate the first year, but some lie dormant for one or more 
years. The period of time during which disseminules remain 
viable is extremely diverse, though it is much longer, as a rule, 
for seeds than for spores. The great vitality of weed seeds is 
probably due to the vigor of the embryos. 




Fig. 106. — The ecesis of young pines beneath the parent trees. 

280. The effect of habitat. The influence of the habitat upon 
germination is often decisive. Seeds may be carried into a num- 
ber of different formations, any one or all of which may offer 
conditions unfavorable to germination. Habitats are of two 
sorts with respect to the chances of germination, those that are 
new or denuded, and those that bear plants. The probability of 
germination is usually greater in vegetation than in denuded areas, 
chiefly because the surface of such areas is relatively xerophytic. 
On the other hand, the lack of competition in a denuded area 
tends to make final establishment much more certain. 



COMPETITION AM) ECESIS 265 

281. Adjustment to the habitat. Growth and reproduction 

in the new place or habitat depend upon the habitat form, the 
plasticity, and the vegetation form of the plant. Even though 

it may germinate, a typical shade plant, such as I m pattens, will 
not thrive in an open meadow, nor will characteristic sun plants, 
such as most grasses, grow in deep shade. Xerophytes do not 
adapt themselves to hydrophytic habitats, nor hydrophytes to 
xerophytic conditions. However, many mesophytes possess to a 
certain degree the ability to adjust themselves to somewhat xero- 
phytic or hydrophytic situations, while plants of open wood- 
land often invade either forest or meadow. This capability for 
adjustment, i.e., plasticity, is greatest in intermediate species, 
those that grow in habitats not too wet or too dry, too sunny or 
too shaded. It is least in forms highly specialized with respect 
to water content or shade. Hence the rule is that ecesis is con- 
trolled largely by the degree of similarity between the old and 
the new habitat, except in the case of plastic species, which possess 
a wider range of adjustment. 

The vegetation form of the invading species is often of the 
greatest importance in determining whether it will be established. 
It is characterized by modifications which w r ere probably pro- 
duced in the original home by competition, and are of primary 
value in securing and maintaining a foothold. These are prac- 
tically all devices for enabling the plant to persist, such as root, 
rootstock, bulb, tuber, woody stem, etc. They find their greatest 
development among trees and shrubs, and their least among 
annual herbs. 

Experiment 67. Influence of habitat form upon ecesis. Make a culture 
for the study of ecesis by sowing in a meter plot a given number of 
seeds of species representing amphibious plants, sun plants, shade plants, 
and xerophytes, and growing them under mesophytic conditions. Note 
the number of seeds that germinate, and the number of individuals of 
each species that succeed in producing flowers. 

282. Barriers. Any feature of the topography or vegetation 
that restricts or prevents invasion is a barrier. Such features are 
usually permanent, and produce permanent barriers, though a 
barrier is often temporary, existing for a few years only, or for a 
single season. Barriers are distinguished as complete or incom- 
plete with respect to the thoroughness with which they limit 



266 PLANT PHYSIOLOGY AND ECOLOGY 

invasion. They often prevent or decrease migration, but as a 
rule their action is largely confined to ecesis. With respect to 
their nature, barriers are usually distinguished as physical and 
biological. 

283. Physical and biological barriers. Physical barriers limit 
invasion by virtue of some marked physiographic feature, such 
as an ocean, lake, river, mountain range, or desert. Biological 
barriers comprise vegetation, man and animals, and plant para- 
sites. Physical barriers act through their dominant physical 
factors by preventing the ecesis of species coming from very differ- 
ent habitats. A body of water is a barrier to mesophytes and 
xerophytes; deserts set a limit to the spread of mesophytic and 
hydrophytic plants. A mountain range is usually an obstacle to 
migration as well as to ecesis. 

Formations, such as forests and thickets, etc., sometimes act 
as direct obstacles to the migration of tumbleweeds and other 
wind-distributed plants. Their greatest influence in decreasing 
invasion arises from their closed nature. When the competition 
is intense, the invading species, though suited to the general con- 
ditions of the habitat, are usually unable to secure a foothold. 
Man and animals affect migration by the destruction of dissemi- 
nules. They act as a barrier to ecesis whenever they make con- 
ditions unfavorable to invaders, or when they turn the scale in 
the struggle for existence, through grazing, tramping, parasitism, 
etc. The absence of insects necessary for pollination sometimes 
acts as a serious barrier to ecesis. Parasitic fungi affect migra- 
tion in so far as they destroy seeds, or reduce the number pro- 
duced. They may prevent or restrict ecesis by destroying the 
invaders, or by placing them at a disadvantage in the competition. 

284. Influence of barriers. Physical barriers are permanent 
as a rule, while biological ones are often temporary. A forest 
or meadow, which often acts as a barrier to invaders from adja- 
cent vegetation, may disappear as a result of a land-slide, flood, 
or burn, or through the activity of man, and leave an area into 
which plants crowd from every direction. Such barriers as para- 
sites and herbivorous animals may appear or disappear at any 
time. When the physical factors of a habitat form a barrier, 
these may be so modified by climatic changes that they no longer 
act as such. A meadow ceases to be a barrier to prairie xero- 
phytes during a period of unusually dry years. Similarly, dry 



COMPETITION AM) ECESIS 



267 



habitats arc much less effective in checking the spread of meso- 
phytes during wel years or seasons. Many xerophytic habitats, 

dunes, blowouts, gravel slides, prairies, etc., are barriers during 
summer and autumn, but not during spring, when rains are fre- 
quent and the surface remains moist. In the absolute sense no 
barrier is complete, since, at one time or another, practically any 
portion of the earth's surface is capable of supporting some kind 
of vegetation. In connection with the natural spread of terres- 
trial plants, however, it is convenient to distinguish partial barriers 
from complete ones. In this sense large bodies of water and 



1 --' 






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?? '■' ' BvvSJ 






■sS^i 


m^nhii 




mM 






* 



Fig. 107. — An endemic plant, Polemonium speciosum, found only in the 
rock clefts of the highest Colorado peaks, such as Pike's, Gray's, etc. 



mountain ranges furnish the best examples of complete barriers. 
Such barriers bring about the isolation of species, and tend to 
restrict species to a single formation or region. 

285. Distance. Though hardly a barrier in the strict sense, 
distance plays an important part in the amount of invasion. The 
effect of distance is clearly seen in migration, especially in the 
case of a denuded area. The formations which touch the area 
usually furnish more than three-fourths of the species which serve 
to reclothe it. The chief reason for this is that the seeds of the 
adjacent species are not so widely scattered by the time the de~ 



268 



PLANT PHYSIOLOGY AND ECOLOGY 



nuded area is reached, and hence a larger number fall within it. 
A second reason lies in the fact that migrants from more distant 
formations must pass through or over adjacent ones. In this 
case the number of disseminules becomes smaller and smaller 
as the distance increases, and the few invaders from a distance 
reach the denuded area only to find it already occupied. Owing 
to the radiate nature of migration, only a part of the fruits 
of a plant or a group can be carried ordinarily in any one direc- 
tion. The bulk of these fall near the parent, the number de- 




Fig. 108. — A polydemic plant, Frasera speciosa, found in several of the 
mountain formations. 

creasing rapidly as the distance increases, until at a few hundred 
meters or a few kilometers even the last fruits disappear. 

286. Endemism. A species is said to be endemic when it 
occurs in a single region or country, and polydemic when it is 
found in two or more regions. These terms refer merely to posi- 
tion or distribution, and are distinct from indigenous, originating 
in the place where found, and exotic, native in a place other than 
that in which the species is found. A species is best said to be 



COMPETITION AM) ECESIS 269 

endemic when it is found in a single formal ion, and polydemic 

when it occurs in two or more. 

Endemism is duo bo the lack of migration, or the presence of 
barriers. Plants which are extremely immobile, e.g., those that 
migrate slowly by underground offshoots, are often endemic. 
Many species of alpine regions and of oceanic islands are endemics, 
owing to the check offered by barriers to their spread. Immobile 
species which are surrounded by barriers are almost inevitably 
endemic. However, the most immobile plant may be scattered 
widely by new or unusual agencies, or the most formidable barrier 
may be sometimes overcome, especially through the activities of 
man. Endemism is also a direct result of evolution, since new 
forms as a rule are endemic at first. Whether they remain en- 
demic depends upon their mobility and upon the presence of 
barriers to migration or ecesis. An endemic may also arise in 
rare instances by the disappearance of a polydemic in all regions 
or formations but one, owing to competition or to changed physi- 
cal conditions. 

A species usually becomes polydemic by virtue of being carried 
into two or more different formations or regions and becoming 
established there. A plant may also be polydemic in conse- 
quence of having originated in two or more places. 

Experiment 68. Barriers and endemism. Make a list of the species 
found in two or more neighboring ravines, separating those found in 
one ravine from those that occur in all. Make a similar list for the 
intervening hills or ridges. Point out the action of barriers in several 
of the most striking instances. 



CHAPTER XIV 
INVASION AND SUCCESSION 

287. Invasion. The movement of one or more plants from 
one area into another and their establishment in the latter con- 
stitute invasion. Invasion may concern an individual or a num- 
ber of individuals of one or more species. It may take place 
between plant groups of any kind, though it is most conspicuous 
between different formations. Invasion is a regular occurrence 
between adjacent formations, but it also takes place into re- 
mote ones, as a result of long carriage by wind, water, birds, rail- 
roads, or vessels. 

Migration and ecesis are both necessary factors in invasion. 
The former carries the spore or seed into the area to be invaded. 
In ecesis the spores or seeds germinate and grow and the new 
plants become established after more or less adjustment. In 
practically all terrestrial forms invasion is possible only when 
migration is followed by ecesis. In the case of aquatic forms 
distributed regularly by water, ecesis is of little or no importance, 
and migration often becomes identical with invasion. 

288. The manner of invasion. Since ecesis is largely con- 
trolled by competition, the manner and amount of invasion are 
determined by the presence or absence of vegetation. Soils with- 
out vegetation are either new or denuded, while areas covered 
with vegetation are either open or closed. Each kind of area 
presents different conditions to invaders. Naked habitats, i.e., 
rocks, talus, dunes, are usually invaded with great difficulty, 
because of their xerophytic character, and the slowness with which 
invaders secure a foothold, in spite of the lack of competition. 
Denuded habitats usually offer the best opportunities for invasion. 
They ordinarily contain a large number of disseminules ready to 
spring up when the original occupants are destroyed. The sur- 

270 



INVASION AM) SI CCESSION 



71 



face offers good conditions for germination, and there is no com- 
petition to decrease the chances of ecesis. open formations are 
readily invaded, since competition is slight, and physical factors 
rarely extreme. On the other hand, closed formations reduce 
invasion greatly, owing to the intensity of the competition which 
newcomers must meet. 

Invasion takes place by the entrance of new individuals. The 
manner depends largely upon the nature of the seed or fruit. 
The invasion of wind-distributed species with comate or winged 
seeds or one-seeded fruits is by individuals as a rule. Migrants 



- .... - 




1 

■ 




"^Sj^ppP ^Rn 


• 


isyss 


* 


0' 


-fi&MM 









Fig. 109- — Invasion of the shore of a lake, almost wholly by rosette plants 
which have migrated from the mass in the background. 

in which the disseminule is a many-seeded fruit or plant tend 
to produce a group of invaders. The accidents of migration some- 
times bring a few separate seeds together into one group, or scatter 
those of a many-seeded fruit, but these are relatively rare excep- 
tions to the rule. This difference in the manner of entrance is 
an important factor in alternation, and is the basis of the two 
types of abundance, i.e., copious and gregarious. 



272 PLANT PHYSIOLOGY AND ECOLOGY 

The seeds of two or more species which are intermingled or 
adjacent are frequently carried into a new area at the same time. 
This process of mass invasion is characteristic of new habitats 
and of the transition areas between formations or regions. The 
best examples of it are found in valleys, where there is a definite 
line of movement. 

289. Invasion at different levels. The invasion of a forma- 
tion or some part of it may occur at three different levels: (1) at 
the level of the facies, (2) below the facies, (3) above the facies. 
This is determined by the relative height of invaders and occu- 
pants. The level at which invasion occurs determines the whole 
future of the formation, as well as the structural modification 
of the plants concerned. The entrance of invaders of the same 
general height as the facies regularly produces a mixed formation. 
Facies and invaders are rarely so equally matched that they 
remain in permanent equilibrium. Any slight advantage of the 
one over the other tells sooner or later, and invader slowly yields 
to occupant, or the reverse. 

When invasion takes place below the facies, the invaders are 
either slowly adopted into the formation, or gradually disappear. 
In either case there is usually little change in the formation, and 
its structure is modified slightly if at all. If the invaders over- 
top the facies in any considerable number, the entire formation 
is more or less modified, or it disappears entirely, as is the rule 
in succession. 

290. Kinds of invasion. Continuous invasion occurs between 
formations or areas more or less alike, and is usually mutual. 
There is an annual movement from each one into the other, and 
often a forward movement through each, resulting from the in- 
vaders of the previous year. Most invasion is of this sort. Tran- 
sition areas are good examples of continuous invasion, which 
is usually mutual also. Intermittent invasion commonly arises 
through migration to a distance. The movement is more or 
less accidental, and may never recur. Such invasion is relatively 
infrequent, but it is often striking, owing to the fact that the 
invader often wanders far from the original home. 

Invasion is complete when the number of invaders is so great 
that the original occupants finally disappear. Such invasion 
occurs in many ruderal or weed formations, and is typical of 
many successions. It is ordinarily the result of continuous in- 



LWASIOX AND SUCCESSION 273 

vasion. Partial invasion takes place when the number of in- 
vaders is sufficiently small that they may be adopted into the 
formation without changing it materially. Partial invasion is 
more frequent though less conspicuous than complete invasion. 

When an invader persists for a few years and then disappears, 
invasion is temporary. This is usually the case in the earlier 
stages of succession which replace each other more or less rapidly. 
Permanent invasion occurs when a species becomes permanently 
established in a more or less stable formation. It is characteristic 
of the grassland and forest stages of succession. 

291. Indigenous and derived species. Species or forms that 
have arisen within a formation, or have belonged to it since its 
origin, are indigenous. Species that have invaded the formation 
at a later date are derived. The latter are termed vicine when 
they are fully established invaders from adjacent formations or 
regions, and adventitious when they have come from distant for- 
mations. Derived species w^hich are unable to establish them- 
selves permanently are adventive. 

Experiment 69. Invasion. Stake a quadrat in an area denuded by 
flooding, or in consequence of cultivation or building. Trace the en- 
trance of invaders, and determine their source in so far as possible. 
Stake a second quadrat in the transition area between two formations, 
and determine to which formation the various species belong. 

292. Succession. The process by which a series of invasions 
occurs in the same spot is termed succession. This is always 
the result of invasion, but not all invasion leads to it. The num- 
ber of invaders must be large enough or their effect sufficiently 
controlling to bring about the gradual disappearance of the orig- 
inal occupants. Partial or temporary invasion rarely causes 
succession. On the other hand, it is the regular consequence 
of complete and permanent invasion. Succession depends pri- 
marily upon invasion in such quantity and of such kind that the 
reaction of the invaders upon the habitat prepares the way for 
further invasion. The characteristic presence of stages, or for- 
mations, in a succession is due to invasion and reaction. In a 
denuded habitat, for example, migration from the adjacent for- 
mations is constantly taking place, but a relatively small num- 
ber of migrants become established. These reach a maximum 
development in size or number, and in so doing react upon the 



274 



PLANT PHYSIOLOGY AND ECOLOGY 



habitat so that new migrants find increasingly favorable con- 
ditions. These in turn attain their maximum, and cause the 
gradual disappearance of the species of the first stage. At the 
same time they prepare the way for the plants of the succeeding 
stage. This successive replacement of one set of plants by another, 
which gives the name to this process, is due partly to the reaction 
of the plants upon the habitat, and partly to the competition 
between them. The close competition between individuals of 
the same or similar species causes them to displace each other, 




Fig. 110. — Stages of a primary succession. Owing to the partial breaking 
down of the rock, four stages which ordinarily follow each other are 
here found side by side. The solid surface is covered with crustose 
lichens, the crumbling edge shows foliose lichens, beyond these are 
mosses, and in the new soil mat and rosette herbs are appearing. 

though they at the same time permit the entrance of invaders 
of a different character. 

293. Kinds of succession. A particular succession is started 
by the physical or biological disturbance of a habitat or forma- 
tion. With respect to the initial cause, successions are normal or 
anomalous. A normal succession begins with a bare habitat, and 
ends in a stable formation. An anomalous succession is one in 
which the facies of the usual final stage are replaced by other 



INVASION AND SUCCESSION 275 

species, or one in which the development departs radically from 
the normal course. Normal successions occur everywhere, anoma- 
lous ones are infrequent. 

Normal successions may arise upon soils or habitats newly 
formed, or upon those laid bare by the destruction of the plants 
upon them. New soils are more or less unfavorable to ecesis. 
The>- possess few or no seeds, and many changes are necessary 
before a stable formation can take possession. Consequently a 
succession in a new soil takes place slowly and exhibits many 
stages. It is termed a primary succession. Denuded soils as a 
rule offer much better conditions for ecesis owing to the action 
of the previous vegetation. Dormant seeds are more or less 
abundant. Revegetation takes place rapidly and shows few stages. 
Such a succession is termed secondary. 

A normal succession is usually perfect, i.e., its stages occur 
in the usual sequence from initial to final formations without an 
omission. Imperfect succession results when one or more of the 
stages is omitted anywhere in the course, and a later stage appears 
before its turn. It may develop at any time when a new or de- 
nuded habitat is so surrounded by other vegetation that the usual 
invaders are unable to enter, or when the abundance, nearness, or 
mobility of certain species enables them to take possession before 
their turn. 

294. Primary successions. These arise on newly formed soils 
or upon those exposed for the first time. In general they are 
characteristic of mountain regions, where weathering is the rule, 
and of lowlands and shores where sedimentation is constantly 
occurring. The physical processes that bring about the for- 
mation of new soils are: (1) elevation, (2) volcanic action, 
(3) weathering. The first two are infrequent and often local, 
and hence need not be considered here. 1 Weathering is prac- 
tically universal, and in connection with the disposition of the 
weathered material furnishes the basis for distinguishing new 
soils. In accordance, the latter are divided into the following 
groups: (1) residuary soils, which are formed in the position 
occupied by the original rock, (2) colluvial soils, due to the action 
of gravity upon the weathered material, (3) alluvial soils, those 
arising by deposit in water, (4) seolian soils, which are formed 
or deposited by winds, (5) glacial soils, due to the action of gla- 

1 Research Methods. 241. 



276 



PLANT PHYSIOLOGY AND ECOLOGY 



ciers. The primary successions upon colluvial, alluvial, and seo- 
lian soils are by far the most frequent and important. 

295. Succession in colluvial soils. Colluvial deposits are 
found at the base of cliffs, ledges, and mountain sides. The for- 
mation of talus, i.e., the mass of coarse irregular material at the 
base, is due to the action of gravity upon the fragments of rock 
split off by disintegration. Decomposition appears later as a 
secondary factor. The fragments forming the talus are extremely 




3 



m 



Fig. 111. — A mat formation, the first stage of an alpine colluvial succession. 



variable in size, but they agree in their angular shape. Talus may 
originate from any kind of rock, the nature of the latter deter- 
mining the size of its particles. Gravel slides differ from ordinary 
talus in consisting of more uniform particles, worn round by 
slipping down the slope in response to gravity and surface wash. 

The character of the successions in talus depends upon the 
kind of rock in the latter. If the rock is igneous or metamorphic, 
decomposition is slow and the resulting soil is easily dried out. 
The corresponding successions consist of many stages, and the 
formations are for a long time open and xerophytic. In talus 



. INVASION AND SUCCESSION 277 

formed from sedimentary rocks decomposition is usually much 
more rapid, and the successions are simpler and more mesophytic. 
296. Succession in alluvial soils. Alluvial soils are formed 
when any obstacle retards the movement of water. This de- 
creases its ability to carry sediment and causes the deposit of 
all or part of its load. Alluvial soils consist of more or less rounded, 
minute particles mingled with organic matter. Such deposits 
are common at the mouth of streams or rivers and in the valleys 
flooded by them. Alluvium is also formed by the filling of ponds 




Fig. 112. — A meadow formation, the last stage of the alpine colluvial 



succession 



through surface wash and of lakes through the deposit of mate- 
rial by streams entering them. It occurs along coasts where 
bays and inlets are slowly converted into marshes, in consequence 
of being shallowed by the material washed in by waves and tides. 
Two kinds of alluvial deposits are distinguished: (1) those 
black with organic matter and little disturbed by water, and 
(2) those of a light color, which are constantly swept by the 
waves. The corresponding successions are radically different. 
In the first the pioneer vegetation is hydrophy tic ; in the second 



278 



PLANT PHYSIOLOGY AND ECOLOGY 



it is xerophytic. Both become finally more or less mesophytic, 
but in one this takes place by decreasing water content and in 
the other by increasing it. 

297. Succession in aeolian soils. Wind-borne soils are rep- 
resented chiefly by inland and coastal dunes. These consist of 
rounded sand particles of almost uniform size, although this 
varies greatly in dunes of different ages. The reaction of the 
pioneers upon dunes plays an important part in building them 
up, by virtue of binding and holding the shifting soil. A dune 




Fig. 113. — First stage of an aeolian succession, sand dunes of Cape Henry. 



succession usually starts with xerophytes and terminates in 
mesophytic meadows or forests. Inland dunes occur in dry 
regions, and their successions are xerophytic throughout. 

298. Secondary successions. Practically all successions on 
denuded soils are secondary. Exceptions occur only when the 
action of the denuding force is so intense that an entirely new 
soil is exposed, as in landslides. The great majority of secondary 
successions owe their origin to erosion, floods, or the activities 
of man. They occur ordinarily upon soils of medium water con- 



INVASION AND SUCCESSION 



279 



tent. Such soils usually contain considerable organic matter 
and a large Dumber <A dormant seeds. The successions upon 
them consist of relatively few stages, and arc usually mesophytic 
in character. Secondary successions may arise in eroded, flooded, 

or drained soils; they may be caused by subsidence or landslides, 
or by animals or man. 

299. Succession in eroded soils. Habitats are ordinarily 
eroded by the action of water. Sands and gravels are readily 
worn away, owing to their lack of cohesion, while loam and clay 
are easily eroded only on slopes. In the former the extreme 
porosity and slight capillarity of the soil result in a low water 
content. In the finer soils the water content is also low, on 
account of the excessive run-off. The action of wind in eroding 
soils which bear vegetation is not general. It is found to some 




Fig. 114. — Secondary succession in a flooded area. The plants of the alpine 
meadow have been drowned out, and the soil entirely occupied by one 
?pecies, Bistorta bistortoides. 

extent in dunes, and is frequent in sandhills, where it produces 
blowouts. 

The early stages of successions in eroded soils are usually com- 
posed of xerophytes. In loose soils these are forms capable of bind- 
ing the soil particles together, thus preventing wash and increasing 
the accumulation of fine particles, especially of organic matter. 
In compact soils the pioneers not only decrease erosion, but also 
increase the water content bv retarding run-off. 



280 



PLANT PHYSIOLOGY AND ECOLOGY 



300. Succession in flooded soils. Floods are confined largely 
to the valleys of streams and to coasts. Flood waters spread 
out over the lowlands of the valley, forming a flood plain. The 
effect of the overflow is to destroy or to place at a disadvantage 
all the plants of the flood plain that are not hydrophytes. At the 
same time a thin layer of fresh silt is deposited upon the floor 
of the valley. Mesophytic species are washed away by erosion, 
or are destroyed by immersion in the water. After a flood which 
destroys the vegetation, algae, liverworts, and mosses come in 




Fig. 115. — The fireweed, Chamcenerium angustifolium, which forms the first 
herbaceous stage of a burn succession in the Rocky Mountains. 

quickly, as pioneers of the succession. These are followed the 
same year or the next one by weeds, which after a few years are 
displaced by the original species. 

301. Succession due to man. Man destroys vegetation 
through fires, lumbering, and cultivation, as well as by drainage 
and irrigation, though the latter usually modify the water con- 
tent merely. Such activities are almost universal. The result- 
ing successions depend not only upon the cause, but also upon 
the region in which it operates. Man by his activities originates 



INVASION AM) SUCCESSION 



281 



an immense Dumber of successions, among which those of burned, 
lumbered, or cultivated areas are far the most frequent. 

302. Succession in burned areas. From the nature of the 
vegetation, tires are of little significance in open formations, such 
as deserts, wastes, etc. In grassland the living parts are under- 
ground at the time when fires ordinarily occur. In consequence 
the annual burning of prairie or meadow disturbs the formation 
very little, and no succession results. All formations composed 
of woody plants, e.g., forests and thickets, are seriously injured 




Fig. 116. — An aspen forest, the typical stage of the burn succession. 



by fire. A severe general fire destroys the vegetation completely. 
A local fire destroys the growth in a restricted area, while a super- 
ficial burn removes the undergrowth and hastens the disappear- 
ance of the weaker trees. A local fire prepares the way for a 
succession in which the invaders come largely from the original 
formation, especially when this encloses the burned area more 
or less completely. When a particular formation is destroyed 
wholly or in large part, the first stages of the new vegetation are 
formed by invaders from the adjacent formations. The sue- 



282 PLANT PHYSIOLOGY AND ECOLOGY 

ceeding stages contain an increasing number of species found 
in the original forest, until, in the final formation, the original 
facies have returned. The reconstruction of a mesophytic forest 
regularly takes place by means of mesophytes, owing to the fact 
that the change in soil is slight. 

303. Succession in lumbered areas. Lumbering often re- 
sults in complete or nearly complete destruction of the vegeta- 
tion through removal, or through the action of erosion upon the 
exposed surface. In the first case a short mesophytic succession 
results. In the second the succession is long and complex, pass- 
ing through decreasingly xerophytic conditions to a stable meso- 
phytic forest. Where a forest is cut over for certain species 
alone, the undisturbed trees usually take full possession. In some 
instances a newcomer usurps the first place, while in others the 
original species ultimately return. 

304. Succession by cultivation. The clearing of forests and 
the "breaking" of grassland for cultivation destroy the original 
vegetation. The temporary or permanent abandonment of cul- 
tivated fields then permits the entrance of weeds, which are the 
pioneers of new successions. This occurs annually in fields after 
harvest, the same species reappearing year after year. In fields 
that lie fallow for several years or are entirely abandoned, the 
first ruderal plants are displaced by newcomers, or certain ones 
become dominant at the expense of the others. In a few years 
these are replaced by invaders from adjacent formations, and 
the field is ultimately reclaimed by the original vegetation, unless 
this has entirely disappeared from the neighborhood. 

Other activities of man, such as the construction of buildings, 
roads, railways, canals, etc., remove the native vegetation and 
make room for the rapid development of weed formations 
In and about cities, where the original formation has entirely 
disappeared, such initial stages persist as permanent formations. 
Elsewhere the usual succession takes place, and the ruderal vegeta- 
tion is finally replaced by the original one. In mountain and 
desert regions, where ruderal plants are rare or lacking, their place 
is taken by native species of large seed production and much 
mobility. These are gradually replaced by other native species 
of less mobility but greater persistence. 

305. Reactions of plants upon the habitat. A succession 
starts wherever an area is unoccupied, denuded, or otherwise 



INVASION AND SUCCESSION 283 

changed, bo thai fairly complete and continuous invasion may 
take place in it. Its continuance is largely due to the reaction 
which each of its stages exerts upon the physical factors of the 
habitat. The reactions thus produced are as follows: (1) influ- 
ence upon weathering, (2) binding seolian soils, (3) reduction of 
run-off and erosion, (4) filling with silt and plant remains, (5) 
enriching the soil, (6) exhausting the soil, (7) modification of 
atmospheric factors. 

Plants influence weathering by hastening disintegration or 
decomposition, or more rarely by protecting the rock surface 
from the action of weather. This last effect is found in some 
degree where lichens or mosses cover rocks completely, though 
they serve at the same time to decompose the surface. Herbs, 
and especially shrubs and trees, hasten disintegration through 
the growth of roots into cracks and clefts. When present in 
masses, they increase the amount of water, and hence promote 
the action of the latter. The shifting soils of dunes, blowouts, 
etc., are inhabited chiefly by sand-binding plants, mostly per- 
ennial grasses and sedges. These usually have masses of fibrous 
roots, long, erect leaves, and vigorous rootstocks capable of push- 
ing up rapidly through a covering of sand. They react by fixing 
the sand with their roots, thus preventing it from being blown 
away. They also catch the shifting particles among their stems 
and leaves, which also serve to accumulate vegetable remains. 

In habitats subject to erosion, plants delay run-off and pre- 
vent the formation of rills pf sufficient size to erode the soil. Such 
plants are usually perennial grasses or composites with well- 
developed roots. On sand and gravel slopes the loose texture 
of the soil leads to the production of sand-binders and of mats 
and rosettes, which are especially effective in preventing the 
slipping of the soil. In hydrophytic habitats the plants check 
the movement of the water. In consequence they cause the 
deposition of part or all of the sediment carried by it, and they 
then retain and fix the particles deposited. The continuance of 
this action, together with the accumulation of plant remains, gradu- 
ally fills up the habitat. The enrichment of the soil by the decay 
of the plants growing upon it is a reaction that is found in some 
measure in all formations. The reverse process, the exhaustion 
of the soil, is largely confined to cultivated fields. Only a few 
doubtful cases of such action occur in nature. In lavered for- 



284 PLANT PHYSIOLOGY AND ECOLOGY 

mations the reaction of the taller plants in reducing the sun- 
light to shade, and in modifying humidity, temperature, and 
wind, determines the final course of the succession. 

306. The laws of succession. The following arrangement of 
the principles which govern succession furnishes a helpful sum- 
mary of the course of development. 

I. Cause. The initial cause of a succession is the formation or 

appearance of a new habitat, or a striking change in an 
existing one. 

II. Reaction. Each stage of a succession reacts upon the 

habitat in such a way as to produce conditions more or 
less unfavorable to itself, but favorable to the invaders 
of the next stage. 

III. Mobility and nearness 

1. The pioneers of a succession are those species near at 
hand, or most mobile. 

2. The number of migrants from any formation into a 
habitat varies inversely as the square of the distance. 

3. The pioneers usually come from two or more different 
formations, since most formations contain some very 
mobile species. 

4. The plants of the initial stage are usually algae and 

fungi, because of their minute spores, composites and 
grasses on account of their very mobile fruits, or ruderal 
plants owing to their large seed production. 

IV. Ecesis. 

1. The migrants into a new, denuded, or greatly modified 
habitat are sorted by ecesis into three groups : (1) those 
that are unable to germinate or grow, and soon die; 
(2) those that grow normally under the conditions 
present; (3) those that pass through one or more of 
the earlier stages in a dormant state, and appear in a 
later stage of the succession. 

2. Whenever ruderal plants are present, they furnish a 

large number of the pioneers, on account of their ready 
ecesis. In other regions subruderal native species play 
this part. 

3. Annuals and biennials are characteristic of the earlier 

stages of secondary successions, on account of their 
great seed production and ready ecesis. 



INVASION AND SUCCESSION 285 

4. In layered formations sun plants appeal- before shade 
plants, but yield to them, except when they belong 
to tlu 1 primary layer. 

5. Excessive seed production and slight mobility lead to 
the imperfect ecesis o( individuals in dense stands, and 
hence usually produce groups that are temporary. 

6. Each pioneer produces about itself a tiny area of ecesis 
and stabilization, which may be entered by its own 
offspring, by the seeds of its fellows, or by invaders 
from a distance. 

7. Species that propagate by offshoots or produce immobile 
fruits in small number usually establish themselves 
readily, because the offspring appear within the area 
reacted upon by the parent forms. 

V. Stabilization. 

1. The universal tendency of vegetation is toward stabiliza- 
tion. 

2. The ultimate stage of a succession is determined by the 
dominant vegetation of the region. Lichen formations 
are often final in polar and niveal zones. Grassland 
is the final vegetation for plains and alpine stretches, 
and for much prairie, while forest is the last stage for 
all mesophytic habitats. 

3. Grasslands or forests usually terminate successions, 
hence they are most frequent in regions showing few 
physiographic changes. 

4. The end of a succession is largely brought about by the 

progressive increase in competition, which makes the 
entrance of invaders more and more difficult. 

5. Stabilization radiates outward from the pioneer plants 
or masses. The movement of offshoots is aw T ay from 
the parent mass. The chances of ecesis are greatest 
near its edges in a narrow area in which the reaction 
is felt, but competition is not so intense. 

VI. General laws. 

1. The stages or formations of a succession are distinguished 

as initial, intermediate, and ultimate. 

2. Initial formations are open, ultimate formations are 
closed. 

3. The number of species is small in the initial stages. It 



286 PLANT PHYSIOLOGY AND ECOLOGY 

attains a maximum in the intermediate ones, and again 
decreases in the ultimate formation, on account of the 
dominance of a few species. 

4. The normal sequence of vegetation forms in succession 
is: (1) algae, fungi, and mosses, (2) annuals and bi- 
ennials, (3) perennial herbs, (4) bushes and shrubs, 
(5) trees. 

5. The number of individuals of a species increases con- 

stantly to a maximum for each stage, and then gradu- 
ally decreases as the next stage develops. 

6. A secondary succession does not begin with the initial 
stage of the primary one that it replaces, but usually 
at a much later stage. 

7. At present successions generally tend to end in meso- 
phytic formations, grassland, or forest, though many 
remain xerophytic or hydrophytic. 

8. The operation of succession must have been essentially 
the same during the geological past that it is to-day. 

307. The study of succession is carried on by the use of 
instruments for determining the physical factors of the habitat 
and the reaction of the stages upon them, and by the use of quad- 
rat, transect, and migration circle to discover and record the 
changes in structure during the various stages. It is usually 
impossible to apply these methods to the initial stage of a suc- 
cession and then to the succeeding stages, owing to the length 
of time taken by the development. A few secondary successions 
run their course within a few years, but ordinarily the period is 
much longer, especially in primary successions, where it may 
reach several hundred years. Since it is rarely possible to 
follow a succession from beginning to end, it is necessary to em- 
ploy indirect methods of determining its course. The three 
methods used for the purpose are: (1) the method of alternating 
stages, (2) the relict method, (3) the experimental method. 
By the combined use of these, it is possible to reconstruct a suc- 
cession with something of the same detail and accuracy that 
could be obtained by following it from beginning to end. 

308. Method of alternating stages. The same succession often 
starts at the same time or at different times upon several areas 
of similar nature. When the start is made at different times, 
the various areas show different stages in development. If one 



INVASION WD SUCCESSION 



28: 



area shows :in initial stage and another ;m intermediate one, 
the development of the former reveals the history of the inter- 
mediate stage, while that of the latter indicates the stages passed 
through to reach the ultimate one. Hence from a number of 
areas it is possible to work out in a few years the probable se- 
quence of stages in a succession that requires several decades 



MS***:* 



6-S . '.-■ ,"£k*. 




Fig. 117. — Decade method of counting the annual rings of a tree or shrub 
to determine the age ot a successional stage or formation. Check pins 
are inserted in every tenth ring. 

or more for its development. Such a method is not final, but 
it is the only feasible one for most successions. 

309. The relict method. When one stage yields to another, 
some species persist for a longer or shorter period. Some last 
through the next stage, while a few may persist through several 
successive stages. When the stages to which such relicts belong 



288 PLANT PHYSIOLOGY AND ECOLOGY 

are known, their number and position often make it possible to 
throw much light upon the previous course of development. In 
the majority of cases the relict is not modified, and is readily 
recognized as belonging properly to a previous stage. This is 
true of herbs in all the stages of grassland and in the initial ones 
of forest succession. The herbs and shrubs of the earlier stages, 
which persist in the final forest formation, are necessarily modi- 
fied. Their abundance and position, however, usually make it 
clear that they are relicts. 

The lifetime of the forest and thicket stages of succession is 
found by counting the annual rings of trees and shrubs. This 
important method may be readily employed in woody formations 
where stumps abound or a fire has occurred. 

Experiment 70. The study of a secondary succession. Select an 
abandoned field or a denuded area for study. List the species that are 
present, indicating their relative importance. Determine in so far as 
possible the formations from which the invaders have come. Examine 
similar areas, especially those that show other stages, and by means of 
comparison indicate the general course of succession. 

Chart a permanent quadrat that has been previously established, 
and compare the chart with those of former years. 



CHAPTER XV 
ALTERNATION AND ZONATION 

310. The relation between alternation and zonation. The 

structure of all formations, as well as their arrangement into 
vegetation, rests upon two principles, zonation and alternation. 
Both of these are concerned in the structure of a formation. In 
some cases zonation predominates to such an extent that alter- 
nation is subordinated or becomes inconspicuous. More often 
the reverse is true. The alternation of various groups is so marked 
that zonation is completely obscured. Because of its striking 
character, zonation is usually more conspicuous than alterna- 
tion. The latter, however, is more fundamental and more uni- 
versal. 

311. Alternation. This principle deals with the occurrence 
of a formation at different places in a region or of a plant group 
or species at separate points in a formation. It covers all re- 
sponses of vegetation to the unlikeness of the many parts of the 
earth's surface. Alternation stands in sharp contrast to zona- 
tion, since it is caused by a lack of symmetry in habitats. It 
is found in vegetation areas and plant groups of every rank. 
The breaking up of vegetation into formations is a striking example 
of alternation. It is also found within every formation, where 
it is represented by consocies, societies, communities, and families, 
and gives endless variety to its structure. In brief, alternation 
deals with the structures and causes which give to vegetation 
its typical and universal lack of uniformity. 

312. Causes of alternation. The primary cause of alternation 
is unlikeness in the physical factors of various formations or 
areas of the same formation. This condition is universally pres- 
ent in vegetation; it occurs within zones as well as in formations 
that lack them. Alternation is due also to competition and to 
invasion, and naturally any two or all of these causes may act 

289^ 



290 



PLANT PHYSIOLOGY AND ECOLOGY 



together to produce it. Migration carries seeds into the differ- 
ent areas of a formation or into different formations, with little 
respect to the physical nature of these. The physical differences 
between the various areas causes some of the migrants to be 
established in some places and not in others. Other migrants 
establish themselves in another series of like areas, and so on. 
Competition often hinders or prevents the ecesis of a species in 
an area in which the physical factors are otherwise favorable, 
and thus produces alternation independently. Invasion frequently 




Fig. 118. — Alternation of spruce forests and gravel slides in Engelmann 
Canon above Manitou. 

carries plastic species into a number of dissimilar habitats, or 
causes one species to appear in one or more similar areas, and a 
similar or corresponding species in the remaining areas of the 
series. In new or denuded areas invasion and physical factors 
act together to produce alternation, while in closed vegetation 
competition is a factor which operates independently, or in con- 
junction with the habitat. 

313. Alternation due to ecesis. When seeds are carried into 
a number of more or less diverse areas or formations, alternation 



ALTERNATION AND ZONATION 291 

does not arise if the physical factors permit ecesis in all of them. 

This is a rare occurrence, however, and in the vast majority of 
cases ecesis is possible only in those areas closely resembling the 
original home. In consequence the same species, together with 

its neighbors, which necessarily possess the same ability of adjust- 
ment, tends to recur in all similar areas of the formation. The 
same is true on a larger scale of the formations of a region. Exam- 
ples of the same formation recur more or less regularly in the 
same kind of habitat, alternating with other formations which 
occur in habitats of another kind. From this it is evident that 
the type of alternation due to ecesis is a fundamental and universal 
feature of vegetation. 1 

314. Alternation due to competition. The alternation pro- 
duced by ecesis in similar areas is often affected by competition. 
The number and kind of individuals vary in the several areas. 
In some competition is much more intense than in others, con- 
sequently reducing the number of individuals of invader or occu- 
pant, or eliminating one or the other completely. In minor areas, 
in which the physical factors are little if at all different, plants 
persist more readily in areas with slight competition than in those 
where the latter is intense. As a result different groups appear, 
each one tending to recur in those spots where competition is 
least unfavorable to it. 

315. Kinds of alternation. Alternation involves two ideas, 
viz., the alternation of different species or formations with each 
other and the alternation of the same species or formation in 
similar but separate situations. This is the result of a lack of 
symmetry in a habitat or region, in consequence of wdiich adja- 
cent areas are dissimilar and remote ones often similar. The 
same species, group, or formation is said to alternate between 
two or more similar situations, while different species, groups, or 
formations alternate with each other, occurring in situations differ- 
ing in ecesis or competition. From the nature of alternation the 
two cases always occur side by side. 

Alternation is distinguished as normal, numerical, or corre- 
sponsive. The general nature of the first has just been indicated. 
Numerical alternation occurs when a species varies greatly in 
abundance and importance in the various areas in which it is 
found. In corresponsive alternation a species, group, or for- 
1 Research Methods, 283. 



292 



PLANT PHYSIOLOGY AND ECOLOGY 



mation is represented in one or more areas suitable to it by a 
similar or corresponding species, group, or formation. 

Numerical and corresponsive alternation are variations of the 
normal process. They arise out of slight differences in physical 
factors, or the course of competition, or out of migration from 
the surrounding vegetation. 

316. Normal alternation of formations, consocies, etc. The 
alternation of formations or of minor areas is especially character- 
istic of greatly diversified regions, such as mountains. It is 




Fig. 119. — Normal alternation of consocies of the foothill thicket forma- 
tion near Colorado City. Quercus is found on the black soil, Cercocarpus 
on the limestone soil, and Rhus in the valleys. 



naturally much less conspicuous in lands with a more uniform 
surface. A xerophytic formation alternates from ridge to ridge, 
a mesophytic one from one valley to another. Aquatic vegeta- 
tion alternates from pond to pond, or from stream to stream. 
The appearance of new or denuded soils upon which successions 
are established is the most important source of the alternation of 
formations. This is true in general of other causes of succession, 
such as erosion, flooding, burning, and cultivation, especially 
when they occur in areas physically similar. 



ALTERNATION AND ZONATTON 293 

In an extensive formation the .same consociea alternates 
between those areas that are similar. When the formation is 
broken up and occurs here and there in separate examples, a 
consociea often alternates from one to another of these. 

317. Normal alternation of species. The alternation of species 
is the most typical feature of formations. The areas of a habitat 
which show minor differences in physical factors or in compe- 
tition are occupied by individuals of one or more species able 
to adjust themselves to such differences. Individuals of such 
species tend to recur in all areas essentially similar, the inter- 
vening areas being occupied by species more or less different. 
Individuals that are scattered alternate as well as those that 
grow in groups, but their alternation is necessarily much less con- 
spicuous. In habitats not too heterogeneous a number of species 
are sufficiently adjustable so that they occur throughout the 
entire formation. In the prairie formation Festaca, Koelera, 
Panicum, and Andropogon occur practically throughout, except 
in the moist ravines which are really meadows. Astragalus, Pso- 
ralea, Erigeron, and Aster grow on slopes and crests, but they are 
much more abundant in certain situations. Other plants. Lomo- 
tium, Meriolix, Anemone, Pentstemon, etc., recur in similar situ- 
ations upon different hills. Lomatium alternates between sandy 
or sandstone crests; Meriolix and Pentstemon occur together 
upon dry upper slopes, while Anemone alternates between slopes 
and crests. 

318. Numerical alternation. All species that alternate show 
a variation in abundance from one area to another. Frequently 
the difference is slight, and can be determined only by the use 
of the quadrat. Sometimes the variation is so great that a facies 
is reduced in number until it is less important than certain prin- 
cipal or secondary species. A principal species often undergoes 
a similar reduction in importance. This arises from the fact that 
the similar areas are sufficiently different to affect the abimdance 
without entirely suppressing the species. This result is due 
partly, and sometimes wholly, to competition. It is especially 
noticeable in places where a species is placed at an increasing 
disadvantage. Numerical alternation is often due to recent in- 
vasion, in consequence of which the species has not yet had time 
to reach its normal abundance. Astragalus crassicarpus. for 
example, grows on nearly all the slopes of the prairie formation. 



294 PLANT PHYSIOLOGY AND ECOLOGY 

On some it is as abundant as the facies, while on others it is repre- 
sented by a few scattered individuals. Such alternation is much 
more striking in separate examples of the same formation, par- 
ticularly when the abundance of a facies is normal in one and 
extremely reduced in another. This is a matter of much impor- 
tance in the study of formations, since a separate consocies may 
otherwise be mistaken for the formation itself. 

319. Corresponsive alternation. Owing to the accidents of 
migration and competition, similar areas within a habitat are 
not always occupied by the same species or group of species. A 
species found in one area may be replaced in another by a differ- 
ent one of the same or another genus. Such genera and species 
are termed corresponding. They must be essentially alike in 
habitat form, i.e., in response to the habitat, though they may 
be entirely unrelated systematically. A good example of corre- 
sponding alternation is found upon exposed sandy crests of the 
prairie formation where Lomatium occurs upon some, but is repre- 
sented upon others by Comandra or Pentstemon. 

Experiment 71. Alternation of species. Select several species of a 
heterogeneous formation, such as a hilly prairie, for study. Note the 
places at which each species occurs. From the physical nature and the 
location of the various places, indicate whether the alternation of each 
species is due to ecesis and competition or to accidents of migration. 

320. Zonation. Zonation is the common response of plants 
to the way in which physical factors are distributed through a 
habitat or a series of them. In nearly all habitats one or more 
of the physical factors present decrease gradually in passing 
away from the point of greatest intensity. The result is that 
the plants of the habitat arrange themselves in more or less defi- 
nite belts about this point, their position being determined by 
their relation to the factor concerned. Most formations show 
zonation of some kind, though the zones are often incomplete or 
obscure through various causes. In many cases, when the gen- 
eral structure of the formation reveals no zones, it will be found 
that some of the species are arranged zonally. As a rule, zona- 
tion is more characteristic of vegetation, as a whole, than of the 
formation. This is seen in the zones of continents, though these 
are often interrupted, owing to climatic and physiographic differ- 
ences, so that they are not always continuous. 



ALTERNATION AND ZONATION 



l> ( .»:, 



321. Zones due to growth. The causes that produce /oiks 
are either biological or physical. The firsl are connected with 
some feature of the plant, the second with the physical factors 
of the habitat. Biological causes arise from the method of growth, 
the manner of migration, or the reaction of the species upon the 
habitat. The formation of circles as a result of radial growth 
is a very common occurrence. It is well known in the case of 
the "fairy-rings" of certain mushrooms. It is also found in a 
Large number of molds, mildews, and other fungi, especially the 




Fig. 120. 



-Bog, meadow, thicket, and forest zones at the upper end of 
Lake Moraine. 



foliose lichens. The thalloid liverworts show a similar radial 
growth. Flowering plants, and many mosses also, furnish excel- 
lent examples in those species that form mats, turfs, or carpets. 
Growth results in zonation only when the older central por- 
tions of the individual or mass die away, leaving a gradually 
widening belt of younger plants or parts. This seems to be due 
in part to the greater age of the central portion, and in part to 
the successful competition of the young, actively growing parts 
for the water and nutrient material of the soil. Such miniature 



296 



PLANT PHYSIOLOGY AND ECOLOGY 



zones of growth do not increase in size until they make the zones 
of formations, but they serve as examples of the action of growth 
in zonation. 

322. Zones due to migration and ecesis. The growth of run- 
ners or rootstocks away from the plant or mass in all directions 
is a very effective means of migration. The seeding of the plants 
thus carried away from the central mass is most certain at the 
edge of the newly occupied area. The circle thus becomes larger 




Fig. 121. — A growth zone of Muhlenbergia gracilis, produced by the radial 
growth of the original mat and the dying of the older parts. 

year by year. Sooner or later the younger and more vigorous 
circumference becomes a more or less complete zone. This is 
due to the reaction of the central individuals upon the habitat, 
so that they are readily replaced by invaders, or to their increas- 
ing age and dying out. 

323. Zones due to reaction. Certain reactions of plants upon 
habitats produce zonation. The zones of fungi seem to be caused 
by the exnaustion of the organic matter present. In the mats 
of mosses and flowering plants it is probable that the continued 



ALTERNATION AND ZONATION 297 

reaction of the centra] part has something to do with its disappear- 
ance. The reaction of a forest or thicket, or even of a layer of 
herbs, plays an important part in producing zones. The factor 
chiefly concerned here is light. The intensity is greatest at the 
edge of the formation, and also just below the primary layer. 
It decreases toward the center of the forest and toward the ground. 
In response to these changes, zones appear in both directions. 
The lateral zones are more or less incomplete, and are only due 
in part to differences of light. The vertical zones or layers are 
characteristic of forests and thickets, and are controlled wholly 
by differences in light intensity. 

324. Zones due to physical factors. The physical causes of 
zonation are by far the most important. They arise from differ- 
ences in water, temperature, and light. The great zones of vegeta- 
tion are due to water and temperature differences. In a par- 
ticular region or habitat, variation of water content and humidity 
are the most important, while light acts only in the case of forests 
and thickets. Physical factors produce zonation in a habitat 
or a series of habitats, when there is either a gradual and cumu- 
lative or an abrupt change in their intensity. Gradual slight 
changes are found in single habitats; abrupt marked changes 
in a series of them. 

The change in a decisive factor takes place in all directions 
from the area of greatest intensity, making the habitat more or 
less symmetrical with respect to the factor concerned. If the 
area of greatest intensity is linear, the shading out takes place 
in two directions. The resulting symmetry is bilateral, a con- 
dition found along streams. On the other hand, a central in- 
tense area shades out in all directions, giving rise to radial sym- 
metry, such as is found in ponds, lakes, etc. The close con- 
nection between the two kinds of symmetry becomes evident 
where a stream broadens into a lake, where a mountain ridge 
breaks up into isolated peaks, or a peninsula or landspit is cut 
into islands. The line that connects the points of accumulated 
or abrupt change in the symmetrical area is a stress line or ecotone. 
Such lines are usually well marked between formations, espe- 
cially where the medium changes, as between a pond and a prairie. 
They are less evident within formations. In the one case the 
ecotone separates two distinct series of zones, and in the other 
merely two different zones of the same formation. 



298 PLANT PHYSIOLOGY AND ECOLOGY , 

325. Physiographic symmetry. The symmetry of a habitat 
depends primarily upon the distribution of water in it, and this 
is greatly affected by soil and physiography. Decisive differences 
in soil rarely occur within a single habitat, though this is often 
the case in a series. The strikingly zonal structure or arrange- 
ment of habitats is nearly always due to differences in water con- 
tent produced by physiographic factors, slope, exposure, surface, 
and altitude. All of these have a pronounced influence upon 
water content and humidity. Consequently, wherever appre- 




Fig. 122. — Physiographic symmetry shown by the valley of Bear Creek 
near Colorado Springs. The bilateral zones of Populus angustifolia 
are indistinct, owing to the narrow stream, but the thicket zones are 
marked. 



ciable differences in physiography occur, they produce areas of 
excess or deficiency in water content about which this factor 
varies symmetrically. Peaks and hills are typical examples of 
areas of deficiency; ponds, lakes, and oceans of areas of excess. 
When such areas are extreme in character and close to each other, 
the resulting zonation is marked. When they are moderate, 
particularly if they are widely separated, the zones produced are 
obscure. 

326. Symmetry in vegetation. The response of vegetation to 
habitat is so exact that physiographic symmetry everywhere 
produces a vegetational symmetry, which is expressed in zones. 



ALTERNATION AND ZONATION 299 

In consequence /ones are regular features of vegetation. The 

zonal arrangement of format ions is usually evident, but the zones 
of a formation are often obscured and sometimes lacking. The 
latter is regularly the case in a uniform area, such as a shallow 
pond or a new soil. 

Zones are obscured in several ways. The plants are some- 
times too scattered to make the response to physiographic sym- 
metry evident. The alternation of conspicuous species not only 
interrupts zones, but often it also completely hides the zonation 
of species of lower habit. The ecotones of one factor may run 
at right angles to those of another, and the resulting series of 
zones may obscure each other. A physiographic feature such 
as a hill may have its symmetry interrupted by ridges or ravines, 
which deflect the zones downward or upward, or cause them to 
disappear altogether. 

327. Kinds of zonation. Two kinds of zonation are dis- 
tinguished with respect to the direction in which the controlling 
factor changes. When this is horizontal, as with water content 
and temperature, zonation is lateral. If the direction is vertical, 
as in the case of light, zonation is vertical. There sometimes 
exists a close connection between the two in forests, where the 
secondary layer of small trees and shrubs is continuous with a 
belt of trees and shrubs around the central nucleus and the lower 
layers of bushes and herbaceous plants with similar zones still 
further out. Lateral zonation is radial when the habitat or 
physiographic feature is more or less circular in form, and it is 
bilateral when the latter is elongated or linear. Vertical zona- 
tion is unilateral, i.e., the zones extend in but one direction. 

328. Radial zonation is typical of elevations and depressions, 
especially mountain peaks, islands, lakes, ponds, etc. The zones 
of peaks are ordinarily quite perfect. They are due largely to 
temperature, though humidity also plays a part. The zonation 
of islands, hills, etc., is produced by water content. The zones 
of islands are often regular and complete, while those of hills 
are more often incomplete or obscure. Prairies and steppes do 
not show a series of zones, but their hills and ridges are more 
or less zoned. Ponds and lakes usually show complete zones, 
except in ponds so shallow that the ordinary marginal zone is 
able to extend over the entire bottom. 

The line between an elevation and a depression, i.e., the edge 



300 



PLANT PHYSIOLOGY AND ECOLOGY 



of the water-level, is a sharply defined ecotone. It separates 
two series of zones, each of which constitutes a formation. One 
of these is regularly hydrophytic, the other usually mesophytic. 
The line between the two can rarely be drawn at the water's edge, 
as this is not constant, owing to waves, tides, or periodical rise 
and fall. There is in consequence a more or less variable transi- 
tion zone of amphibious plants, which belong properly to the 
hydrophytic formation. Many forest formations serve as a center 
about which are arranged several incomplete zones. 




Fig. 123. — Radial zonation of Sparganium angustifolium in one of the 
Seven Lakes. 



329. Bilateral zonation differs from radial only because it is 
produced by linear elevations and depressions rather than cir- 
cular ones. With this difference, the zones of ranges and ridges 
correspond exactly to those of peaks and hills, while the same 
relation is evident between the zones of streams and of lakes 
and ponds. The ecotones are identical except in form; they 
are linear in one, circular in the other. Incompleteness is more 
frequent in bilateral zonation, though this is due largely to the 
length of bilateral zones. 



ALTERNATION AM) ZONATION 



301 



330. Vertical zonation is peculiar in that then 4 is no ecotone 
present, on either side of which zones arrange themselves with 
reference to the factor concerned. This is due to the fact that 
the controlling factor is light, which falls upon the habitat in such 
fashion that it decreases in but one direction, i.e., downward. 
Vertical zones appear in bodies of water on account of the absorp- 
tion of light by the latter. The most characteristic zones occur 
in forests, where the primary layer of trees acts as a screen. The 
density of this screen determines the number of zones or layers 




Fig. 124. — Bilateral zonation of cord grass, Spartina cynosuroides , 
along a ditch. 



present. In extreme cases the foliage is so dense that the light 
beneath is insufficient even for mosses and lichens. As a rule, 
how T ever, there are one or more layers present. 

A vertically zoned forest shows a complete series of reac- 
tions. The primary layer determines the amount of light, heat, 
and water for the subordinate layers in general. Each of the 
latter further modifies the amount for those below it, the ground 
layer being subject in some degree to the reaction of every layer 
above it. The lower layers also influence the upper by reacting 



302 



PLANT PHYSIOLOGY AND ECOLOGY 



upon the habitat through absorption, transpiration, decompo- 
sition, etc. 

331. Vegetation zones. Zonation is the most striking fea- 
ture of the vegetation of continents, a fact well illustrated by the 
vegetative covering of North America. It is produced by a 
gradual variation in both water content and temperature, though 
the former is much the more important. In accordance with the 
decrease of water content and temperature northward, three 




Fig. 125. — Vertical zonation, or layering, in a spruce forest. 

primary belts of vegetation stretch across the continent from 
east to west. These are forest, grassland, and polar desert. The 
first is further divided into the secondary zones of broad-leaved 
evergreen, deciduous and needle-leaved forests. At right angles 
to this symmetry due to temperature and water lies a second one 
produced by water alone. In response to this, forest belts touch 
the oceans, but give way in the interior to grasslands, and these 
to deserts. The interference of these two series of zones has 



ALTERNATION AND ZONATTON 303 

produced the primary features of the vegetation of North America. 
Tropical forests occur where heat and water are excessive, deserts 

where either is unusually deficient, deciduous and coniferous 
forests where the water content is relatively high, and prairie 
and plains where it is relatively low. 

This primary arrangement is modified by the disturbing effect 
of three continental mountain systems. The Appalachian system 
is not sufficiently high to produce a pronounced effect upon humidity 
and rainfall. In consequence forests extend far beyond it into 
the interior before giving way to prairies and plains. On the 
other hand, the influence of the Rocky Mountains and the Sierra 
Nevada is marked. The latter rise to a great height relatively 
near the coast, and condense upon their western slopes nearly 
all of the moisture brought from the Pacific. The Rocky Moun- 
tains have a similar effect upon the drier winds from the east, 
and the two systems in consequence enclose a parched desert. 
All three systems cany the formations of the north far beyond 
their normal southern limit, owing to the low temperatures that 
prevail in high altitudes. The alpine grasslands of the Sierra 
Nevada and the Rocky Mountains are a southward extension of 
arctic grasslands, and the belts of coniferous forests along the 
slopes of the three systems are similar extensions of northern 
forests. 

Experiment 72. Zonation of pond and meadow formations. Select for 
study a pond of some extent and depth. Note the various zones of 
vegetation and list the species of each. Make a map showing the general 
outline of the pond, and indicate the limits of the various zones of pond 
and meadow. 



INDEX 



abnormal succession, 274 
absolute humidity, 25 
absorption, 38 

relation between, and transpira- 
tion, 38 
abundance, 205 
Acacia, 162 
Acer, 242 

negundo, 126 
achene, 132 
Aeon it urn. 120 

columbianum, 57 
acospores, 243 
Adansonia, 117 
adaptation, 4, 190 

origin by, 192 

to excessive water supply, 152 

to light, 171,181 

to small water supply, 144 

to water, 144 

types produced by, to water, 155 
adjustment, 4 

abnormal, 5 

kinds of, 5 

normal, 5 

to contact, 141 

to gravity, 135 

to the habitat, 265 

to light, 71 

to shock, 142 

to temperature, 90 

to water, 38 
adventitious, 273 
adventive, 273 

aeolian soils, succession in, 278 
sestival flowers, 130 
Agave, 160, 162 

americana, 147 
age of plants, 112 
agents of migration, 244 
aggregation, 220, 237 

mixed, 239 

simple, 237 
Agropyrum caninum, 207 
aianthous flowers, 130 



air-spaces, 59, 175 
Alisma, 167 

alkalies, influence of, 147 
Allionia, 157, 182, 184 

linearis, 86, 178, 173, 179, 182, 184 

nyctaginea, 182 
Allium, 133, 207 

recurvatum, 134 
allogamy, 124 

alluvial soils, succession in, 277 
alpine plants, 27, 98 
alternating stages, 286 
alternation, 220, 289 

causes of, 289 

corresponsive, 291, 294 

due to competition, 291 

due to ecesis, 290 

kinds of, 291 

normal, 291, 292, 293 

numerical, 291, 293 

relation between, and zonation, 289 
altitude, 22, 27, 63 

effect on light, 76 
amides, 101 
Amorpha, 227 

amphibious plants, 153, 166 
ancestral factor, 186 
Andropogon, 226, 293 

consocies, 226 
Androsace, 120, 207 

diffusa, 180 
anemochores, 245 
Anemone, 121, 207, 228, 242, 293 
anemophilous flowers, 125, 126 
animals, in migration, 245 
Antennaria, 159, 207, 227 
anthotropism, 121 
Apocynum androsawiifolium, 145 
Aguilegia ccerulea, 250 
Arab is fendleri, 157 
Aracece., 120 
Aragallus lamberti, 207 
Arctestaphylus uva-ursi, 119, 157 
Ariscema, 120, 126 
Aristida, 244 

305 



306 



INDEX 



Artemisia, 159, 160, 162, 165 

frigida, 207 
artificial selection, 196 
Asclepias, 242 
Ascophora, 105 
asparagin, 101 
Asparagus, 163 
aspects, 223 
Aster, 227, 246, 293 
Astragalus, 226, 293 

crassicarpus, 293 
autogamy, 124, 127 

direct, 127 

indirect, 127 
autotrophic plants, 104 
autumn aspect, 224 
available water, 9, 14 

determination of, 13 
awned disseminules, 243 

Baccharis, 162 

Bacon, on evolution, 188, 192 

Bacteriacece, 243 

Bahia dissecta, 150, 164 

barriers, 265 

biological, 266 

influence of, 266 

physical, 266 
belt transect, 211 
berry, 131 
Betula, 126, 242 
Bicuculla, 120 
Bidens bigelovii, 184 
Bignonia, 126 
bilateral zo nation, 300 
biological barriers, 266 
biotic factors, 5 
bisexual crosses, 198 
Bistorta bistortoid.es, 279 
blastochores, 247 
bog plants, 168 
'bog xerophytes," 19, 168 
bolochores, 247 
Botrydium, 39 
Bouteloua, 226 

Bouteloua-Koelera-consocies, 226 
Brassica alba, 42 
Brauneria, 242 
brotochores, 246 
Bryophyllum, 162 
buds, 114 
bulblets, 115 
bulbs, 115 

burned areas, succession in, 281 
Bursa, 178, 182 

bursa- pastoris, 179 

Cactacece, 164 
calcium, 19, 101 
Callitriche, 125, 167 



Callitriche autumnal 's, 167 

bifida, 149, 168 
Calluna, 162 
Caltha, 169 

leptosepala, 166 
Calypso borealis, 60, 126 
calyptra, 41 
cambium, 41, 51, 53 
Campanula aparinoides, 121 

petiolata, 224 

rotundifolia, 121 
capillarity, 16, 18, 20, 55 
Capnoides aureum, 179 
capsule, 132 
carbonates, 19 

carbon diox de absorption and diffu- 
sion, 81 
Car ex, 242 
carphospores, 242 
carpotropic movements, 132 
Castilleia Integra. 206, 207 
cedar apples, 5 
Cenchrus, 243 
centrospores, 243 
Cer ostium, 243 
Ceratophyllum, 125, 168 
Cercocarpus, 292 
chaffy disseminules, 242 
Chamamerium angustifolium, 123, 136, 

137, 140, 195, 197 
changes of epidermal cells, 147 
chart quadrat, 204, 205 
chemosynthesis, 100 
chlorenchym, 57, 86 

changes in, 150, 172 
chlorides, 19 
chlorophyll, absorption spectrum, 79 

influence of, 80 

nature of, 79 

production of, 79 
chloroplasts, 177 

influence of, 171 

influence of light on number and 
position of, 85 

number and position, 86 
chlorovaporization, 63 
chresard, 9, 13, 14 
Circcea, 120 
circumnutation, 141 
cladophyll form, 163 
classification by habitats, 231 

developmental, 233 

of formations, 230 

regional, 234 
Claytonia, 131 
cleistogamous flowers, 123 
Clematis, 120, 243 . 
climate, 27 

climatic factors, influence of, 24 
clinometer, 23 



INDEX 



307 



clitochores, 2 16 

closed formations, 235, -71 

cog psychrometer, 27, 28 

Col I d ia. It. I 

colluvial soils, successioQ in, 27t> 

Coma ml m , 226, 29 I 

comate disseminules, 242 

community, 225, 227, 240 

comospores, 2 1-' 

compass plants, 1 15 

compel it ion. 251 

alternation due to. 291 

between different species, 255 

between parents and offspring, 254 

cultures, 259 

effect oi position. 256 

factors in. 253 

for water and light . 253 

influence of vegetation form and 
habitat form, 250 

law of, 255 

mixed cultures, 259 

nature of, 252 

primary law of, 254 

quadrats, 261 

results of, 25S 

simple cultures, 259 

study of, 259 
consocies, 225 
constancy, 187 

constant and inconstant forms, 186 
contact, 141 
copious, 205 
Corallorhiza , 104, 105 
Corispe.rmum, 238, 245, 252, 262 
conn, 115 

corresponding species, 294 
cortical region, 52 
cover, 22, 24, 27 
creatospores, 243 
Crocus, 131 
crosses, 197 

cross-pollination, 124, 125 
crystallochores, 246 
Cucurbita. 142 

cultivation, succession by, 282 
cultures, competition, 259 
Cupressus, 162 

curvature, region of geotropic, 138 
curves, combinations of, 34 

intervals, 36 

kinds of, 34 

plotting, 35 
cuticle, 57, 147 
cutinized leaves, 157 
Cyperacece, 126, 161, 163 
cytase, 100 
cytoplasm, in growth, 106 

darkness, influence of, 80 



Darwin. 187,251 

and the "( Origin of Species." 188 

evolut ion after. 1 89 

on compel it ion. L95 

on single origin, 199 
Datura, 261 

day-bloomers, 120, 130 
decade method, 287 
decomposil ion, 17 
definite variation, 189 
dehiscent dry fruits, 132 
Delphinium, 120 
denuded quadrat, 204, 209 
derived species, 273 
dermatogen, 39, 41 
developmental classification, 233 
development of the format on, 219 
De Vries, 200 

on mutation, 190, 192 
dew, 21 

diaphragms, 167 
diastase, 100, 103 
dichogamy, 122 
diclinism, 122 

dicotyledons, stem structure of, 52 
diffusion, 48, 84 

in the leaf, 58 
digestion, 99 
diphotic leaves, 174, 182 
diplophyll, 183, 184 
direct factors, 5 
direction of migration, 249 
Discomycetes, 247 
disintegration, 16 

disposition of stamens and pistils, 122 
dissected form, 160 
dissemination, organs of, 241 
disseminules, 242 

position of, 244 
division of labor, 116 
dominant character, 198 
dominant species, 221 
Draba, 228 
drupe, 132 
dry fruits, 132, 247 

ecad, 155, 192, 193 

ecesis, 98, 220, 261, 270, 284 

alternation due to, 290 

factors in, 262 
echard, 9, 14 
Edwinia, 217 
Elymus, 225, 239 

Ehjmus-Muhletibergia-iormcLtion, 225 
endemic, 268 
endemism, 268 
endoderm, 41, 52 
endosmose, 46 
enzymes, 99, 100 
Ephedra, 163 



308 



INDEX 



ephemeral flowers, 123, 130 
epidermal cells, changes of, 147 

layer of root, 42 

region, 52 
epidermis, changes of, 177 

print, 78 

protection of, 144 
epiphytes, 151 
Erica, 162 
Ericales, 162 

Erigeron, 131, 158, 227, 293 
Eriophorum , 242 

pinnatisectus , 161 

speciosus, 178 
eroded soils, succession in, 279 
erosion, 283 
Erysimum, 247 
Erythronium, 120, 229 
Eschscholtzia californica, 121 
establishment, 220 
etiolation, 81 
etiolin, 80 
Euphorbia, 164 
evolution, 185 

after Darwin, 189 

experimental, 201 

fundamental methods of, 191 
exoderm, 41, 52 
exosmose, 46 
exotic, 268 

experimental adaptation to water,153 
experimental evolution, 201 
exposure, 22, 27 

facies, 221 

factors, constant, 31 

in competition, 253 

in ecesis, 262 

variable, 31 
"fairy-rings," 295 
family, 225, 228, 237 
fermentation, 102 
Festuca, 293 
fibro vascular bundles, 51, 84 

system, 52 

system in transport, 51 
flagellate disseminules, 243 
flattened form, 163 
fleshy disseminules, 243 

fruits, 131 
floating plants, 167 
flooded soils, succession in, 280 
flowering, period of, 128 

time of daily flowering, 130 
flowers, zygomorphic, li8 
fluctuating variability, 189 
follicle, 132 
form of leaves, 178 

of stems, 179 
formaldehyde, 82 



formations, classification of, 230 

closed, 231, 235 

development and structure, 219 

historical factor in, 219 

hydrophytic, 232 

maps, 213 

mesophytic, 233 

mixed, 231, 235 

nature of, 215 

open, 231,235 

parts of, 225 

plant, 215 

recognition of , 216 

relation between habitat and, 218 

structure of, 221 

types of, 232 

xerophytic, 233 
formula for vegetation, 205 

for water content, 11 
Fragaria, 212 
Frasera speciosa, 269 
freezing, 96 
fructification, 131 
fruit-distributed plants, 242 
fruits, movements of, 132 
Fuchsia, 137 

functions influenced by temperature, 
91 

light, 71 

of the stem, 54 

of vegetation, 219 
fungi, digestion in, 99 

Galium, 182 
gametophore, 115 
Gaura parviflora, 181 
geitonogamy, 124, 125 
Genista] 162, 163 
Gentiana, 120 
geotome, 10 
geotropism, 135 

cause and reaction, 137 

ecological significance of, 139 
Geranium, 247 
germination, 102 

in ecesis, 263 
Gilia, 120, 160, 162 
glaciers, in migration, 146 
Glceocapsa, 237 
glceospores, 243 
glucose, 82, 84, 101 
Gossypium, 242 
Graminacece, 126, 242 
grass form, 160 
gravity, 108 

in migration, 246 

reaction time, 138 

region of curvature, 138 

relation of plant to, 135 

sensory zone, 138 



INDEX 



309 



gregarious, .0^ 

(irirnli Ha . 1 59 

sguarrosa, 1 58 
growth. I. 106 
amount and rate, 108 

conditions that influence, H )s 

in migral ion, 2 l<i 

of tissues and organs, 106 

regions of greatest growth, 109 

rhythm of, 110 

aone of division, 110 

zone of elongation, 110 

zones due to, 295 

guard-cells, 52 

movements of, lil 
Gyrostachys stricta, 175, 184 

Haberlandt, on transpiration, 67 
habitat, adjustment to, 265 

effect on ecesis, 264 

reactions of plants upon, 282 

relation between formation and ,21 8 

study, method of, 30 
habitat-form, 155, 265 

influence on competition, 256 
habitats, 232 

classification by, 231 
hairs, 57, 148, 175 
Hales, on transpiration, 67 
halophytes, 156 

Helianthus, 159, 163, 165, 182, 227, 
242, 261 

annuus, 182 

pumilus, 176 
heliophytes, 183 
heliotropic plants, 88 
Helleborus, 62 
hemeranthous flowers, 130 
Henslow, on adaptation, 190, 192 
heterotrophic plants, 104 
Heuchera, 217 
Hibiscus trionum, 31 
Hippurus, 167 

vulgaris, 167 
historical factor in vegetation, 219 
Holacantha, 164 
holard, 9, 14, 16 
holophytes, 104 
homogamous flowers, 122 
hooked disseminules, 243 
Hordeum, 229 
hour readings, 33 
humic acids, 169 
humidity, 7, 24, 63 

effect of climate and habitation, 27 

influence of pressure and physiog- 
raphy on, 26 

influence of temperature and wind , 
25 

measurement, 27 



humidity readings, 29 

tables. 30 
humus, 20 
hybridation, origin by, 197 

hybridization, 124 
hydroehores, 245 
hydrophilous flowers, 125 
hydrophytes, 155, 165 
hydrophytic formations, 232 
hydrotropism, 43, 44 
hygroscopic fruits, 247 
hysterophytes, 104 
nutrition of, 104 

imbibition, 44, 45 

Impatiens, 126, 247, 265 

increase of water supply, 151 

indefinite variations, 189 

indehiscent dry fruits, 132 

indigenous, 268, 273 

indirect factors, 5 

insectivorous plants, digestion in, 99 

invasion, 270 

at different levels, 272 

complete, 272 

continuous, 272 

intermittent, 272 

kinds of, 272 

manner of, 270 

partial, 273 

permanent, 273 

temporary, 273 
invertase, 100 
Ipomoea, 121 
Iris, 120, 226 
iron, 79 
isolation, 199 
isophotic leaves, 174, 182 

J uncus, 161, 162, 163 

Kcelera, 226, 293 
Kuhnistera, 227 
Kuhnia glutinosa, 213 

Laciniaria, 227, 228 
Lactuca, 242 

scariola, 145 
Lamarck, on evolution, 188, 192 
Lanate leaves, 159, 175 
law of competition, 254, 255 

of evolution, 185 

of simple aggregation, 239 
laws of succession, 284 
layers 229 

leaf position, decrease of water loss 
through, 145 

print, 78, 178 

structure, 56 

xerophytes, normal, 157 



310 



INDEX 



leaf xerophytes, other, 159 

xerophytes, types of, 156 
leaves, outline, size and thickness of, 

178 
Lecanora, 215 
legume, 132 
Leguminosce, 105 
Lemna, 167, 168 
Lemnacece, 167 
Lepidium, 229 
leucoplasts, 80 

act-on of, 85 
levels, invasion at different, 272 
lichens, 105 
light, 5, 63 

action on chloroplasts, 85 

adaptation to, 171 

amount absorbed , 77 

comparison with standard, 75 

competition for, 253 

functions, 71 

intensity, 72, 75, 76 

intensities in forests, 230 

measurement of, 72 

readings, 74 

reception and absorption, 77 

relation of organs to, 171 

relation of plants to, 71 

standard, making a, 73 

stimuli, 72 

types of leaves, 182 
Liguliflorce, 121 
line transect, 210 

Linnaeus, on genera and species, 187 
lipase, 100 
list quadrat, 204 
Lithospermum, 120, 228 
Lomatium, 226, 293, 294 
loment, 132 
Lonicera, 126 
lophospores, 242 
Ludwigia, 169 

lumbered areas, succession in, 282 
Lygodesmia, 163 

Machairanthera, 178 

aspera, 125, 130, 179 
magnesium, 19 
major quadrat, 204 
maltose, 101 
Malva arborea, 67 
man, agent in migration, 246 

succession due to, 280 
mapping quadrat charts, 206 
maps, formation, 213 
Matthews, 189 
maximum temperature, 95 
Mentzelia, 239 
Meriolix, 293 
meristem, 106, 107 



Mertensia, 158 

linearis, 158, 164 
Mesembryanthemum, 160 
mesenchym, 41 
mesophytes, 155, 164 
mesophytic formations, 233 
Micrampelis, 141, 142 
migration, 99, 220, 237, 240, 270 

agents, constant, 248; intermit- 
tent, 248 

agents of, 244, 248 

circle, 212 

determinate, 249 

direction of, 250 

indeterminate, 250 

modifications for, 242 
Mimosa pudica, 142 
minimum temperature, 95 
mixed aggregation, 239 
mixed formations, 235 
mixotrophic plants, 104 
mobility, 241 

and nearness, 284 
modification, 4 

modifications for migration, 242 
modifying factors, 15 
Moneses uniflora, 128 
monoclinism, 122 
monocotyledons, 53, 170, 172 
Monotropa, 104 
monotropic, 201 
mortar fruits, 247 
motility, 241 
mountain zones, 303 
movements of fruits, 132 

of stems and leaves, 87 
Muhlenbergia, 162, 165 

gracilis, 296 
mutants, 196 
mutation, 190 

origin by, 196 
mutualism, 104 
mycorhiza, 105 
Myriophyllum, 168 

natural selection, 189, 191, 196, 198 
nature of competition, 252 
Naudin, 189 
nectar flowers, 127 
neeole form, 161 
night-bloomers, 120, 130 
nitrates, 19 
nitrogen, 18, 101 

bacteria, 105 
Nobbe, on root areas, 44 
non-available water, 9, 14 
normal alternation of formations, 
292 

alternation of species, 293 

leaf xerophytes, 157 



INDIA 



311 



normal succession, 27 1 

Nostoc, 105 

numerical altemal ion, 203 

nut. 132 

nutrient .silt s, I s . 68 

Nymphcea, 168, 2 12 

oyctanthous flowers, L30 

nyctotropism, 89 

oblate cells, 172 
Odontoglossum, 131 
(Edogonium, 2 13 
Ihera, 100.200 

larnarckiana, 190 
offsets, 115 

offshoot-distributed plants, 242 
Onagra, 120 122, L82, 200 
oncosporcs, 24.3 
open formations. 234, 235, 271 
optimum, 95 
Orchis, 117 

organs of dissemination, 241 
origin by adaptation, 192 

by descent before Darwin, 187 

by hybridation, 197 

by mutation, 196 

by variation, 195 

of new forms, 185 
"Origin of Species," 187, 188, 189 
osmosis, 45 

in root-hairs, 46 
osmotic pressure, 55, 62 
Ostrya, 242 

outline of sun and shade leaves, 178 
Oxalis, 121, 247 
oxides, 19 

Pachylophus hirsulus, 129 
palisade cell, 172 

leaf, 183 

tissue, 57, 151, 176 
Panicum, 293 

parachute disseminules, 242 
parasites, 1C4 

kinds of, 104 
parental care, 116 
Parietaria, 126 
pathology, 5, 6 
Pedicularis procera, 164, 184 
Penieillium, 105 

Pentstemon, 120, 157, 165, 293, 294 
pepsin, 100 
pepo, 132 
periblem, 39, 41 
pericycle, 41 

periodicity of growth, 110 
period of flowering, 128 
permanent quadrat, 204, 208 
perquadrat, 204 
petasospores, 242 



phenology, 131 
PhUotria, 168 
phloem, 1-, 52 

phosphates. 10, 101 
phosphorus, IS, 101 
photometer. 72 
photosynthesis, SI, 104 

chemical changes during, 82 

formuke, 82 

measurement of, 83 

rays active in, 81 
phototropism, 71, 87 
phvad, 155 
phyllode form, 162 
PhysaliSy 242 
physical barriers, 266 

factors, 30, 36 

factors, zones due to, 297 
phvsiographic classification, 233 
Plantago, 126 
plant body, types of, 154 
plant-distributed species, 242 
plant formation, 215 

temperatures, 92 
plasmolysis, 47 
plastic forms, 186 

species, 192 
plasticity, 185, 265 
plerome, 39, 41 
Pleurococcus , 105 
plumed disseminules, 242 
pod, 132 

point readings, 33 
polarity, 108 

Polemonium speciosum, 267 
pollen, amount of, 116 

flowers, 127 

production, 116 

protection by movement, 120 

protection of, 118 

seasonal protection of, 121 

source and distribution of, 124 

structural protection of, 119 
pollination, agents in, 125 

by insects, 126 

kinds of, 124 
polychrome, 201 
polydemic, 268 
polydemics, 184 
polygamy, 122 
polygenesis, 199 
Polygonum, 163 

aviculare, 261 
polytopic, 201 
pome, 132 
Populus, 120, 126 
porosity, 18, 20 
Portulaca, 122 
position, effect on competition, 256 

of disseminules, 244 



312 



INDEX 



Potamogeton, 126 
potassium, 19, 101 
potometers, 64, 67 
precipitation, 21 
pressure, 26 

vegetation, 257 
pre vernal aspect, 224 

flowers, 130 
primary regions, root, 39 

successions, 275 
principal species, 222 
production of pollen, 116 
prolate cells, 172 
propagation, 113 
propagules, 112, 113 

of flowering plants, 114 

stems as, 114 
propulsion, 247 
Proserpinaca palustris, 167 
protandrous flowers, 122 
protection of pollen, 118 

by movement, 120 

seasonal, 121 

structural, 119 
proteids, 101 
protogynous flowers, 122 
protoplasm, selective power of, 48 
Pseudotsuga, 222 
Psoralea, 227, 293 
psychrometer, 27, 28, 29 
pterospores, 242 
Pulsatilla, 243 
pulvinus, 142 
pyxis, 132 

quadrat, 202 

chart, 204, 205 

competition, 261 

denuded, 204, 209 

list, 204 

major, 204 

making charts, 206 

permanent, 204, 208 

tapes, 204 
quadrats, kinds of, 203 

marking out, 204 
Quercus, 184, 292 

novimexicana, 183 

radial zonation, 299 
rainfall, 21 

gauge, 21 

measurement of, 21 
Ranunculus, 167, 169 

delphinifolius , 153, 167 

sceleratus, 149, 153, 155, 194 
reactions of plants upon habitat, 282, 
284 

zones due to, 296 
readings, point and hour, 33 



recessive character, 198 
records, field, 33 
reduced bundles, 58 
reduction of leaf or stem, 146 
regional classification, 234 
relative humidity, 25, 27 
relict method, 287 
reproduction, 112 

sexual, 115 
respiration, 101 
response, functional, 3 

nature of, 3 

structural, 3 

to contact, 141 

to shock, 142 

to water, 7 

to water stimuli, 38 
resting period, 111 
results of competition, 258 
rhizomes, 115 
Rhus, 292 

rhvthm of growth, 110 
roll form, 162 
rolling of the leaf, 145 
root-cap, 40, 41 
root-hairs, 17, 39, 42, 48 

effect of water content upon, 43 

origin and structure, 42 
root pressure, 55 

primary regions, 39 

structure, 40 

system, 40 
root tubercles, 105 
roots, form of, 39 
root stocks, 115 
Roripa americana, 153, 167 
Rosa, 228 
Rumex, 126, 242 

altissimus, 41 
runner, 114 
run-off, 21 
Ruppia, 125 
Ruscus, 163 
rush form, 163 
Rydbergia, 224 

grandiflora, 223 

saccate fruits, 242 
Saccharomyces , 105 
saccospores, 242 
Sachs, starch method, 83 
Sagittaria, 167, 169 

latifolia, 169 
Saint-Hilaire, on evolution, 188, 192 
Salix, 126, 220, 242 
Salvia, 243 
samara, 132 
saprophytes, 99, 104 
sarcospores, 243 
saxifrage, 88 



[NDEX 



313 



scale form. 162 
Bciophytes, 183 
Scirpus, 1 ('».'> 
scutellum of grains, 100 
Scutellaria, 17s, L93 
Beoondary species, 222 

succession, 275, 278 
sedimentation, 283 
Sedum, 160, 162 
Sedum stenopetalum, 160 
seed-distributed plants, 241 
seed, in ecesis, 263 

production, influence of, 243 
seled ion, 196 
self-pollination. 124, 127 
Senecio, 160, 228, 242 

taraxacoiaes, 170 
sensitive plant, 142 
sensory area of tendrils, 141 
serotinal flowers, 130 
sexual reproduction, 115 
shade leaves, 178 

plants, 1G5, 183 

tents, 181 
shock, 142 
Sicyos, 141 
Sieversia, 243 
sieve tissue, 53, 84 
Silene, 243 
silicle, 132 
silique, 132 

Silphium laciniatum, 145 
simple aggregation, 237 

law of, 239 
simultaneous readings, 30, 31, 36 
size and thickness of leaves, 179 
sling psvchrometer, 27, 28 
slope, 22, 27 

influence of, 23 
snowfall, 21 
society, 225, 226 
soil, air-content of, 19 

can, 10 

chemical nature of, 18 

influence on water content, 15 

mechanical analysis of, 20 

properties, determination of, 19 

samples, 10; depth of, 12 

"sourness" of, 19 

structure, 16, 20 

temperatures, 92 

texture, 15, 20 

thermo meter, 92 
soils, origin and structure, 16, 17 
Solidago, 227, 260 

canadensis, 258 

decumbens, 191 

oreoppila, 191 
soluble salts, 18 

absorption of, 48 



soluble salts, influence of, 47 
solutes, 18 
Sparganium. 126 

angttstifohum, 152 
sparse, 205 

Spartina cynosuroides, 301 ' 
spiny disseminules, 243 
Slrirodela, 167 
sponge cell, 172 

leaf, 183 

tissue, 57, 151, 174 
spongophyll, 183, 184 
spore-distributed plants, 241 
Sporobolus, 162 
sporophore, 113, 115 
sports, 189, 196 
spring aspect, 224 
stability, 170, 185 
stabilization, 285 
stable forms, 186 
standard, comparison with, 75 

making a, 73 
Stapelia, 164 
Staphylea, 242 
starch, 82, 84, 85, 100, 103 

determination of, 83 

stations, 31 
staurophyll, 183, 184 
stems, form of, 179 

functions of, 54 

of monocotyledons, 53 

of shade plants, 180 

structure of herbaceous dicoty- 
ledons, 52 

structure, types of, 51 

xerophytes, 162; tvpesof, 162 
Stipa, 45, 162, 226, 244 

consocies, 226 
stimulatory action, 2 
stimuli, external, 3 

internal, 3 

kinds of, 3 

nature of, 1, 2 

nature of light, 72 
stimulus of gravity, 137 

of shock, 143 
stolon, 114 
stomata, 52, 57, 153 

functions, 61 

in sun and shade leaves, 178 

modifications of, 148 

structure and position, 60 
storage leaves, 58 

of food material, 84 

of water. 151 
structure of formations, 219, 221 

of leaf, 56 

of woody stems, 53 
struggle for existence, 251 
subcopious, 205 



314 



INDEX 



subgregarious, 205 
submerged plants, 168 
succession, 273 

abnormal, 274 

by cultivation, 282 

due to man, 280 

in seolian soils, 278 

in alluvial soils, 277 

in burned areas, 281 

in colluvial soils, 276 

in eroded soils, 279 

in flooded soils, 280 

in lumbered areas, 282 

kinds of, 274 

laws of, 284 

method of alternating stages, 286 

normal, 274 

primary, 274, 275 

relict method, 287 

secondary, 275, 278 

study of, 286 
succulent form, 159, 164 
sugar, 103 
sulphates, 19, 101 
sulphur, 18, 101 
sum of temperatures, 97 
summer aspect, 224 
sun leaves, 178 
sun plants, 165, 183 
support, 54 
surface, 22 

influence of, 23 
symbiosis, 104 
symmetry in vegetation, 298 

physiographic, 298 

Tamarix, 162 
Taraxacum, 121, 182, 242 

taraxacum, 182 
Teleonyx jamesii, 88 
temperature, 5, 25 

adjustment to, 90 

effect of low, 98 

favorable and unfavorable, 95 

influence of clouds, winds, etc., 95 

influence on vegetation, 98 

maximum and minimum, 95 

measurement of, 91 

of the air, 91 

of the plant, 92 

of soils, 92 

sum of, 97 

variations of, 94 
tendrils, 141 

sensory area, 141 
Tetraneuris, 159 
Tetraspora, 237 
Teucrium, 120 
texture of soil, 15, 20 
Thalictrum, 178 



Thalictrum sparsiflorum,, 179 

thermometer, 91 

thermotropic plants, 91 

thorn form, 164 

Thuja, 162 

Tilia, 120 

time of daily flowering, 130 

tonic action, 2 

tracheary vessels, 53 

Tradescantia, 122 

transect, 210 

belt, 211 

line, 210 
translocation, 83 
transpiration, 56 

adjustment to, 69 

amount of, 66 

compensation for increased, 68 

influence of physical factors, 63 

measurement of, 64; in field, 65 

relation between absorption and , 68 
transpiring surface, 59 
transport, 50, 54 
Tribulus, 243 
trypsin, 100 
tuber, 115 

turgescent fruits, 247 
turgidity, 48, 61, 68 
types of formations, 232 

of leaf xerophytes, 156 

of leaves as determined by light, 
182 

of plant body, 154 

of stem xerophytes, 162 

produced by adaptation to water, 
155 
Typha, 126, 242 

Ulothrix, 243 
Umbelliferce, 242 
Umbilicaria, 215 
unisexual crosses, 197 
Urtica, 126 
utricle, 132 

Vaucheria, 243 
variability, fluctuating, 189 
variants, 196 
variation, definite, 189 

indefinite, 189 

origin by, 195 
vegetation, development and struc- 
ture, 219 

effect of temperature on, 98 

form, 155, 265 

historical factor in, 219 

influence of vegetation form on 
competition, 256 

methods of studying, 202 

pressure, 257 



INDEX 



315 



\ egetal ion, symmetry in, 298 

zones, 302 
venial flowers, L30 
vert ical zonal ion, 301 
Vida faba, L09, L38 
vicine, _7:> 
\'iold. 2 17 
virgate form. 163 
viscid disseminules, 243 
Vitis, 1 U 

vixgregarious, J05 
Volvocacea , 243 

Wagnera, 239 

steUata, K), 146 
Wallace, L89 

water, 5 
adaptation to, 144 
adjustment to, 38 
available, 9, 14 

capacity, 17 
competition for, 253 
content, 7,8,63 

computation of, 11 

effect of acids and alkalies, 19 

influence of other factors, 9 

influence of precipitation, 21 

influence of soil, 15 

location of readings, 12 

time of readings, 12 
experimental adaptation to, 153 
loss, 177 

decrease by rolling of leaf, 
145 

decrease through leaf position, 
145 
migration, 245 
non-available, 9, 14 
of the habitat, 7 
stimuli, 7 
storage cells, 158, 159 

tissue, 152 



water supply, adaptation to exces- 
sive, 152 
adaptal ion to small, 144 

increase and storage of. 151 
upward movement in plant. 55 

weathering, 1(1, 283 

Wells, L89 

Wiesner, on transpiration, 67 

wilting, 69 

winged disseminules, 242 

wind, 25 

in migration, 245 

witches' brooms, 5 

Wolflia, 167 

wood fibers, 53 

woody stems, structures of, 53 

xenogamy, 124 
xerophytes, 155, 156, 164 

general features of, 156 

leaf, 157 

stem, 162 
xerophytic formations, 233 
xylem, 42, 52 

zonation, 220, 294 

bilateral, 300 

kinds of, 299 

radial, 299 

relation between alternation and, 
289 

vertical, 301 
zones, 212 

due to growth, 295 

due to migration and ecesis, 295 

due to physical factors, 297 

due to reaction, 296 

vegetation, 302 
zoochores, 245 
zoophilous flowers, 125, 126 
Zostera, 125 
zymogen, 100 



AUG 2 1907 



